Primary elections in the United States are elections in which the candidates for a particular office at federal, state or local level are chosen by registered voters in a particular jurisdiction. This includes the presidential primary, which nominates candidates for the presidential election. After the preliminary primary election, a general election is held to fill the office with one of the candidates chosen in the primary election. The United States is one of few countries to select candidates through popular vote in a primary election system; most countries rely on party leaders to vet candidates, as was previously the case in the U.S.  State law, not federal, regulates most aspects of primary (as well as general) elections, and local election officials (county, city, and township) are predominantly responsible for administering them. 
A primary election is an election which meets one of the following conditions:
- An election which is held prior to a general election, as a direct result of which candidates are nominated, in accordance with applicable State law, for election to Federal office in a subsequent election is a primary election.
- An election which is held for the expression of a preference for the nomination of persons for election to the office of President of the United States is a primary election.
- An election which is held to elect delegates to a national nominating convention is a primary election.
- With respect to individuals seeking federal office as independent candidates, or without nomination by a major party (as defined in 26 U.S.C. 9002(6)), the primary election is considered to occur on one of the following dates, at the choice of the candidate:
- The day prescribed by applicable State law as the last day to qualify for a position on the general election ballot may be designated as the primary election for such candidate.
- The date of the last major party primary election, caucus, or convention in that State may be designated as the primary election for such candidate.
- In the case of non-major parties, the date of the nomination by that party may be designated as the primary election for such candidate.
- With respect to any major party candidate (as defined at 26 U.S.C. 9002(6)) who is unopposed for nomination within his or her own party, and who is certified to appear as that party's nominee in the general election for the office sought, the primary election is considered to have occurred on the date on which the primary election was held by the candidate's party in that State. 
In the United States, other types can be differentiated:
- Closed primary. People may vote in a party's primary only if they are registered members of that party prior to election day. Independents cannot participate. Note that because some political parties name themselves "Independent", the terms "non-partisan" or "unaffiliated" often replace "independent" when referring to those who are not affiliated with a political party. 14 states — Delaware, Florida, Kansas, Kentucky, Louisiana, Maine, Maryland, Nebraska, New Jersey, New Mexico, New York, Pennsylvania, Oregon and Wyoming — have closed primaries.  
- Semi-closed. As in closed primaries, registered party members can vote only in their own party's primary. Semi-closed systems, however, allow unaffiliated voters to participate as well. Depending on the state, independents either make their choice of party primary privately, inside the voting booth, or publicly, by registering with any party on Election Day. Twelve states — Alaska, Arizona, Colorado, Iowa, Kansas, Massachusetts, New Hampshire, North Carolina, Rhode Island, Utah, and West Virginia — have semi-closed primaries that allow voters to register or change party preference on election day.  
- Open primary. A registered voter may vote in any party primary regardless of his own party affiliation. Eleven states — Alabama, Arkansas, Georgia, Hawaii, Michigan, Minnesota, Missouri, Montana, North Dakota, Vermont, and Wisconsin — have open primaries.  When voters do not register with a party before the primary, it is called a pick-a-party primary because the voter can select which party's primary he or she wishes to vote in on election day. Because of the open nature of this system, a practice known as raiding may occur. Raiding consists of voters of one party crossing over and voting in the primary of another party, effectively allowing a party to help choose its opposition's candidate. The theory is that opposing party members vote for the weakest candidate of the opposite party in order to give their own party the advantage in the general election. An example of this can be seen in the 1998 Vermont senatorial primary with the nomination of Fred Tuttle as the Republican candidate in the general election. Fred Tuttle was seen as protest vote in the Republican Primary against Jack McMullen, a wealthy, recent transplant from Massachusetts, who resided in an hotel room when decided to run in 1998.
- Semi-open. A registered voter need not publicly declare which political party's primary that they will vote in before entering the voting booth. When voters identify themselves to the election officials, they must request a party's specific ballot. Only one ballot is cast by each voter. In many states with semi-open primaries, election officials or poll workers from their respective parties record each voter's choice of party and provide access to this information. The primary difference between a semi-open and open primary system is the use of a party-specific ballot. In a semi-open primary, a public declaration in front of the election judges is made and a party-specific ballot given to the voter to cast. Certain states that use the open-primary format may print a single ballot and the voter must choose on the ballot itself which political party's candidates they will select for a contested office.
- Blanket primary. A primary in which the ballot is not restricted to candidates from one party.
- Nonpartisan blanket primary. A primary in which the ballot is not restricted to candidates from one party, where the top two candidates advance to the general election regardless of party affiliation. Louisiana has famously operated under this system, which has been nicknamed the "jungle primary".  California has used a nonpartisan blanket primary since 2012 after passing Proposition 14 in 2010, and the state of Washington has used a nonpartisan blanket primary since 2008. 
Primaries can be used in nonpartisan elections to reduce the set of candidates that go on to the general election (qualifying primary). (In the U.S., many city, county and school board elections are non-partisan.) Generally, if a candidate receives more than 50% of the vote in the primary, he or she is automatically elected, without having to run again in the general election. If no candidate receives a majority, twice as many candidates pass the primary as can win in the general election, so a single seat election primary would allow the top two primary candidates to participate in the general election following.
As a result of a federal court decision in Idaho Republican Party v. Ysursa, the 2011 Idaho Legislature passed House Bill 351 implementing a closed primary system. 
When a qualifying primary is applied to a partisan election, it becomes what is generally known as a blanket  or Louisiana primary: typically, if no candidate wins a majority in the primary, the two candidates receiving the highest pluralities, regardless of party affiliation, go on to a general election that is in effect a run-off. This often has the effect of eliminating minor parties from the general election, and frequently the general election becomes a single-party election.
Because many Washington residents were disappointed over the loss of their blanket primary, which the Washington State Grange helped institute in 1935, the Grange filed Initiative 872 in 2004 to establish a blanket primary for partisan races, thereby allowing voters to once again cross party lines in the primary election. The two candidates with the most votes then advance to the general election, regardless of their party affiliation. Supporters claimed it would bring back voter choice; opponents said it would exclude third parties and independents from general election ballots, could result in Democratic or Republican-only races in certain districts, and would in fact reduce voter choice. The initiative was put to a public vote in November 2004 and passed. On July 15, 2005, the initiative was found unconstitutional by the U.S. District Court for the Western District of Washington. The U.S. Supreme Court heard the Grange's appeal of the case in October 2007. In March 2008, the Supreme Court upheld the constitutionality of the Grange-sponsored Top 2 primary, citing a lack of compelling evidence to overturn the voter-approved initiative. 
In elections using voting systems where strategic nomination is a concern, primaries can be very important in preventing "clone" candidates that split their constituency's vote because of their similarities. Primaries allow political parties to select and unite behind one candidate. However, tactical voting is sometimes a concern in non-partisan primaries as members of the opposite party can vote for the weaker candidate in order to face an easier general election.
In California, under Proposition 14 (Top Two Candidates Open Primary Act), a voter-approved referendum, in all races except for that for U.S. President and county central committee offices, all candidates running in a primary election regardless of party will appear on a single primary election ballot and voters may vote for any candidate, with the top two vote-getters overall moving on to the general election regardless of party. The effect of this is that it will be possible for two Republicans or two Democrats to compete against each other in a general election if those candidates receive the most primary-election support.  
In the United States, Iowa and New Hampshire have drawn attention every four years because they hold the first caucus and primary election, respectively, and often give a candidate the momentum to win their party's nomination.
A criticism of the current presidential primary election schedule is that it gives undue weight to the few states with early primaries, as those states often build momentum for leading candidates and rule out trailing candidates long before the rest of the country has even had a chance to weigh in, leaving the last states with virtually no actual input on the process. The counterargument to this criticism, however, is that, by subjecting candidates to the scrutiny of a few early states, the parties can weed out candidates who are unfit for office.
The Democratic National Committee (DNC) proposed a new schedule and a new rule set for the 2008 Presidential primary elections. Among the changes: the primary election cycle would start nearly a year earlier than in previous cycles, states from the West and the South would be included in the earlier part of the schedule, and candidates who run in primary elections not held in accordance with the DNC's proposed schedule (as the DNC does not have any direct control over each state's official election schedules) would be penalized by being stripped of delegates won in offending states. The New York Times called the move, "the biggest shift in the way Democrats have nominated their presidential candidates in 30 years." 
Of note regarding the DNC's proposed 2008 Presidential primary election schedule is that it contrasted with the Republican National Committee's (RNC) rules regarding Presidential primary elections. "No presidential primary, caucus, convention, or other meeting may be held for the purpose of voting for a presidential candidate and/or selecting delegates or alternate delegates to the national convention, prior to the first Tuesday of February in the year in which the national convention is held."  In 2020, this date is February 4.
Candidates for President who seek their party's nomination participate in primary elections run by state governments, or caucuses run by the political parties. Unlike an election where the only participation is casting a ballot, a caucus is a gathering or "meeting of party members designed to select candidates and propose policies."  Both primaries and caucuses are used in the Presidential nomination process, beginning in January or February and culminating in the late-summer political party conventions. Candidates may earn convention delegates from each state primary or caucus. Sitting presidents generally do not face serious competition from their party.
While it is clear that the closed/semi-closed/semi-open/open classification commonly used by scholars studying primary systems does not fully explain the highly nuanced differences seen from state to state, still, it is very useful and has real-world implications for the electorate, election officials, and the candidates themselves.
As far as the electorate is concerned, the extent of participation allowed to weak partisans and independents depends almost solely on which of the aforementioned categories best describes their state's primary system. Clearly, open and semi-open systems favor this type of voter, since they can choose which primary they vote in on a yearly basis under these models. In closed primary systems, true independents are, for all practical purposes, shut out of the process.
This classification further affects the relationship between primary elections and election commissioners and officials. The more open the system, the greater the chance of raiding, or voters voting in the other party's primary in hopes of getting a weaker opponent chosen to run against a strong candidate in the general election. Raiding has proven stressful to the relationships between political parties, who feel cheated by the system, and election officials, who try to make the system run as smoothly as possible.
Perhaps the most dramatic effect this classification system has on the primary process is its influence on the candidates themselves. Whether a system is open or closed dictates the way candidates run their campaigns. In a closed system, from the time a candidate qualifies to the day of the primary, he must cater to strong partisans, who tend to lean to the extreme ends of the ideological spectrum. In the general election, on the other hand, the candidate must move towards the center in hopes of winning.
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