All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives.The very first order of business of the Constitutional Convention, after procedural matters such as electing George Washington its president and passing rules to govern the proceedings, was a speech by Edmund Randolph in which he proposed what came to be known as the Virginia Plan.
The Articles of Confederation had allocated one vote to each state in a single house of Congress. The Virginia Plan created a two-house legislature, the lower house to be elected by the people and the upper house to be elected by the lower house from a pool of people nominated by state legislatures. The two-house Congress would have power to "legislate in all cases to which the separate States are incompetent, or in which the harmony of the United States may be interrupted by the exercise of individual Legislation", veto laws passed by individual states they deemed to contradict the Constitution (referred to as the "articles of Union"), and use force against any state they deemed to be "failing to fulfill its duty under the [Constitution]". An executive would be chosen by the legislature, who - besides normal executive duties - would, along with "a convenient number" of the "National Judiciary... compose a Council of revision" that would review every national law and every vetoed state law, with their own veto power that could be overridden. So powerful was this plan that the following day, Charles Pinckney of South Carolina asked whether Randolph wanted to completely abolish the state governments.
The two-branch structure of Congress would be agreed to without a peep except from Pennsylvania, which Madison attributed to Benjamin Franklin "who was understood to be partial to a single House of Legislation."