The three men took on leading roles at the Constitutional Convention almost as soon as it convened on May 25, 1787. In the first week, Randolph, the 33-year-old Virginia governor, introduced the Virginia Plan, written by Madison, which became the starting point for the new national government. Mason, one of Virginia’s richest planters and a major framer of his home state’s new constitution, was the first delegate to argue that the government needed a check on the executive’s power. “Some mode of displacing an unfit magistrate” was necessary, he argued on June 2, without “making the Executive the mere creature of the Legislature.” After a short debate, the convention agreed to the language proposed in the Virginia Plan: the executive would “be removable on impeachment and conviction of malpractice or neglect of duty” – a broad standard that the delegates would later rewrite.
Mason, Madison, and Randolph all spoke up to defend impeachment on July 20, after Charles Pinckney of South Carolina and Gouverneur Morris of Pennsylvania moved to strike it. “[If the president] should be re-elected, that will be sufficient proof of his innocence,” Morris argued. “[Impeachment] will render the Executive dependent on those who are to impeach.”
“Shall any man be above justice?” Mason asked. “Shall that man be above it who can commit the most extensive injustice?” A presidential candidate might bribe the electors to gain the presidency, Mason suggested. “Shall the man who has practiced corruption, and by that means procured his appointment in the first instance, be suffered to escape punishment by repeating his guilt?”
https://www.smithsonianmag.com/history/inside-founding-fathers-debate-over-what-constituted-impeachable-offense-180965083/
Looks like the argument that impeachment should not be pursued because the Senate wont convict or that reelection would exonerate an impeached President was shot down at the constitutional convention in Philadelphia.
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