Madison understood immediately the force of Mason’s objection, but he had a response—a response in which he described limitations on presidential power that, to our great misfortune, have for too long been forgotten. Was there a danger in giving the president the power to pardon? “Yes,” replied Madison, but there was a remedy for the danger in the Constitution as drafted.

“There is one security in this case to which gentlemen may not have adverted: if the President be connected, in any suspicious manner, with any person, and there be grounds to believe he will shelter him, the House of Representatives can impeach him; they can remove him if found guilty.”

Impeachment, in other words, can start, not when the President has been formally charged with a crime; he can be impeached if there are “grounds to believe” that he might “shelter,” that is to say, protect with a pardon, someone with whom he is connected “in any suspicious manner.”

[link to www.brookings.edu (secure)]

AND IT CAN BE DONE AFTER HIS PRESIDENTIAL TERM.

The House has never impeached, nor has the Senate ever tried, a former President. However, both chambers have previously determined that they retain power to proceed against an executive branch official that has resigned from office. The principal precedent is the 1876 impeachment of Secretary of War William Belknap.Jan 15, [link to crsreports.congress.gov (secure)]