The Connecticut Compromise (also known as the Great Compromise of 1787 or Sherman Compromise) was an agreement that large and small states reached during the Constitutional Convention of 1787 that in part defined the legislative structure and representation that each state would have under the United States Constitution. It retained the bicameral legislature as proposed by Roger Sherman, along with proportional representation of the states in the lower house, but required the upper house to be weighted equally among the states. Each state would have two representatives in the upper house.
On May 29, 1787, Edmund Randolph of the Virginia delegation proposed the creation of a bicameral legislature. Under his proposal, membership in both houses would be allocated to each state proportional to its population; however, candidates for the lower house would be nominated and elected by the people of each state. Candidates for the upper house would be nominated by the state legislatures of each state and then elected by the members of the lower house. This proposal was known as the Virginia Plan.
Less populous states like Delaware were afraid that such an arrangement would result in their voices and interests being drowned out by the larger states. Many delegates also felt that the Convention did not have the authority to completely scrap the Articles of Confederation,  as the Virginia Plan would have.  In response, on June 15, 1787, William Paterson of the New Jersey delegation proposed a legislature consisting of a single house. Each state was to have equal representation in this body, regardless of population. The New Jersey Plan, as it was called, would have left the Articles of Confederation in place, but would have amended them to somewhat increase Congress's powers. 
At the time of the convention, the South was growing more quickly than the North, and Southern states had the most extensive Western claims. South Carolina, North Carolina, and Georgia were small in the 1780s, but they expected growth, and thus favored proportional representation. New York was one of the largest states at the time, but two of its three representatives ( Alexander Hamilton being the exception) supported an equal representation per state, as part of their desire to see maximum autonomy for the states. (The two representatives other than Hamilton had left the convention before the representation issue was resolved, leaving Hamilton, and New York State, without a vote.)
James Madison and Hamilton were two of the leaders of the proportional representation group. Madison argued that a conspiracy of large states against the small states was unrealistic as the large states were so different from each other. Hamilton argued that the states were artificial entities made up of individuals, and accused small state representatives of wanting power, not liberty (see History of the United States Senate).
For their part, the small state representatives argued that the states were, in fact, of a legally equal status, and that proportional representation would be unfair to their states. Gunning Bedford, Jr. of Delaware notoriously threatened on behalf of the small states, "the small ones w[ould] find some foreign ally of more honor and good faith, who will take them by the hand and do them justice."
Elbridge Gerry ridiculed the small states' claim of sovereignty, saying "that we never were independent States, were not such now, & never could be even on the principles of the Confederation. The States & the advocates for them were intoxicated with the idea of their sovereignty." 
On June 14, when the Convention was ready to consider the report on the Virginia plan, William Paterson of New Jersey requested an adjournment to allow certain delegations more time to prepare a substitute plan. The request was granted, and, on the next day, Paterson submitted nine resolutions embodying necessary amendments to the Articles of Confederation, which was followed by a vigorous debate. On June 19, the delegates rejected the New Jersey Plan and voted to proceed with a discussion of the Virginia Plan. The small States became increasingly discontented, and some threatened to withdraw. On July 2, the Convention was deadlocked over giving each State an equal vote in the upper house, with five States in the affirmative, five in the negative, and one divided.
The problem was referred to a committee consisting of one delegate from each State to reach a compromise. On July 5, the committee submitted its report, which became the basis for the "Great Compromise" of the Convention. The report recommended that in the upper house each State should have an equal vote and in the lower house, each State should have one representative for every 40,000 inhabitants,  counting slaves as three-fifths of an inhabitant,  and that money bills should originate in the lower house (not subject to amendment by the upper chamber).
After six weeks of turmoil, North Carolina switched its vote to equal representation per state and Massachusetts abstained, and a compromise was reached, being called the "Great Compromise." In the "Great Compromise," every state was given equal representation, previously known as the New Jersey Plan, in one house of Congress, and proportional representation, known before as the Virginia Plan, in the other. Because it was considered more responsive to majority sentiment, the House of Representatives was given the power to originate all legislation dealing with the federal budget and revenues/taxation, per the Origination Clause.
Roger Sherman and Oliver Ellsworth, both of the Connecticut delegation, created a compromise that, in a sense, blended the Virginia (large-state) and New Jersey (small-state) proposals regarding congressional apportionment. Ultimately, however, its main contribution was in determining the apportionment of the Senate. Sherman sided with the two-house national legislature of the Virginia Plan but proposed "That the proportion of suffrage in the 1st. Branch [house] should be according to the respective numbers of free inhabitants; and that in the second branch or Senate, each State should have one vote and no more."  Although Sherman was well liked and respected among the delegates, his plan failed at first. It was not until July 23 that representation was finally settled. 
What was ultimately included in the constitution was a modified form of this plan, partly because the larger states disliked it. In committee, Benjamin Franklin modified Sherman's proposal to make it more acceptable to the larger states. He added the requirement that revenue bills originate in the house.
The final vote on the Connecticut Compromise on July 16 left the Senate looking to the Confederation Congress. In the preceding weeks of debate, James Madison of Virginia, Rufus King of New York, and Gouverneur Morris of Pennsylvania each vigorously opposed the compromise for this reason.  For the nationalists, the Convention's vote for the compromise was a stunning defeat. However, on July 23, they found a way to salvage their vision of an elite, independent Senate. Just before most of the convention's work was referred to the Committee of Detail, Gouverneur Morris and Rufus King moved that state's members in the Senate be given individual votes, rather than voting en bloc, as they had in the Confederation Congress. Then Oliver Ellsworth, a leading proponent of the Connecticut Compromise, supported their motion, and the Convention adopted the Compromise.  Since the Convention had early acquiesced in the Virginia Plan's proposal that senators have long terms, restoring that Plan's vision of individually powerful senators stopped the Senate from becoming a strong safeguard of federalism. State governments lost their direct say in Congress's decisions to make national laws. As the personally influential senators received terms much longer than the state legislators who elected them, they became substantially independent. The compromise continued to serve the self-interests of small-state political leaders, who were assured of access to more seats in the Senate than they might otherwise have obtained. 
Senate representation was explicitly protected in Article Five of the United States Constitution:
...no state, without its consent, shall be deprived of its equal suffrage in the Senate. 
This agreement allowed deliberations to continue and thus led to the Three-Fifths Compromise, which further complicated the issue of popular representation in the House.
- History of the United States Senate
- Constitutional Convention
- Virginia Plan
- New Jersey Plan
- Three-Fifths Compromise
- "Madison's notes, June 16 1787". Yale Avalon project.
- "Madison's notes, May 30 1787". Yale Avalon project.
- "Madison's notes, June 15 1787". Yale Avalon project.
- "Madison's notes, June 29 1787". Yale Avalon project.
- Yazawa, Melvin (2016). Contested Conventions: The Struggle to Establish the Constitution and Save the Union, 1787–1789. JHU Press. p. 67. ISBN 978-1-4214-2026-4.
- US Constitution.net. "Constitutional Topic: The Constitutional Convention". Retrieved October 17, 2007.
- 1 THE RECORDS OF THE FEDERAL CONVENTION OF 1787, at 489, 490, 551 (Max Farrand ed., 1911)
- 2 THE RECORDS OF THE FEDERAL CONVENTION OF 1787, at 94-95 (Max Farrand ed., 1911)
- See Laurence Claus, Power Enumeration and the Silences of Constitutional Federalism http://ssrn.com/abstract=2837390
- National Archives and Records Administration. "The Constitution of the United states Article V". archives.gov.