On September 4, the Committee on Postponed Matters, named to resolve the convention’s thorniest disputes, had replaced the “malpractice or neglect of duty” standard for impeachment with a much narrower one: “treason and bribery.”
Limiting impeachment to treason and bribery cases, Mason warned on September 8, “will not reach many great and dangerous offences.” To make his case, he pointed to an impeachment taking place in Great Britain at the time—that of Warren Hastings, the Governor-General of India.
Hastings had been impeached in May 1787, the same month the U.S. constitutional convention opened. The House of Commons charged Hastings with a mix of criminal offenses and non-criminal offenses, including confiscating land and provoking a revolt in parts of India. Hastings’ trial by the House of Lords was pending while the American delegates were debating in Philadelphia. Mason argued to his fellow delegates that Hastings was accused of abuses of power, not treason, and that the Constitution needed to guard against a president who might commit misdeeds like those alleged against Hastings. (In the end, The House of Lords acquitted Hastings in 1795.)
Mason, fearful of an unchecked, out-of-control president, proposed adding “maladministration” as a third cause for impeaching the president. Such a charge was already grounds for impeachment in six states, including Virginia.
But on this point, Madison objected. The scholarly Princeton graduate, a generation younger than Mason at age 36, saw a threat to the balance of powers he’d helped devise. “So vague a term will be equivalent to a tenure during pleasure of the Senate,” he argued. In other words, Madison feared the Senate would use the word “maladministration” as an excuse to remove the president whenever it wanted.
So Mason offered a substitute: “other high crimes and misdemeanors against the State.” The English Parliament had included a similarly worded phrase in its articles of impeachment since 1450. This compromise satisfied Madison and most of the other Convention delegates. They approved Mason’s amendment without further debate, 8 states to 3, but added “against the United States,” to avoid ambiguity.
https://www.smithsonianmag.com/history/inside-founding-fathers-debate-over-what-constituted-impeachable-offense-180965083/