A president cannot be impeached after he leaves office. Therefore, were the House of Representatives to impeach the President before he leaves office, the Senate of the United States could not thereafter convict the former President and disqualify him from future public office.

The former President would no longer be incumbent in the Office of the President at the time of the Senate proceeding and would therefore no longer be subject to "impeachment conviction" by the Senate, under the Constitution's Impeachment Clauses.

This is to say that the Senate's power under the Constitution is only to convict (or not) an incumbent president.

The very concept of constitutional impeachment presupposes the impeachment, conviction, and removal from office of a president who is, at the time of his impeachment, incumbent in the Office of President from which he is removed by the impeachment.

The text, structure, and evident purposes of the Constitution's several Impeachment Clauses, all, confirm this understanding.

For example, Article II, Section 4 of the Constitution reads, "The President, Vice President and all civil officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."

And in like constitutional substance, Article I, Section 3 provides in relevant part that "Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States."