In my opinion, impeachment cannot be used against a former President, and definitely not against Members of Congress - there are other constitutional mechanisms for that.

The purpose of an impeachment is to protect the integrity of a public office - to remove an unworthy official from a position of trust. A former President no longer holds public office and is a private citizen, and subject to civil and criminal prosecution once again. Furthermore, Article II, section 4, of the Constitution states the officials subject to impeachment as “the President, Vice President, and all civil officers of the United States . . .“ language which does not encompass former officers of the United States. “Civil officers” has been interpreted to mean officials of the executive or judicial branch of government.

Impeach means “to charge” and the Constitution assigns the role exclusively and only to the House of Representatives. Voting to acquit or convict on any House-adopted Article of Impeachment is the role of the U.S. Senate. If convicted of any article of impeachment, the public official so convicted is then removed from office. As a result, no Senator can start the impeachment process nor can the Senate as a whole accomplish it. It must begin in the House of Representatives.

If for some reason Congress wishes to punish an ex-President they could pass a resolution condemning him with “censure” type language - but that has no official consequence outside of embarrassment. They could also enact a law to reduce the ex-President’s pension, or withdraw his taxpayer-funded office space, staff, or security detail. As with any new law, Congress cannot enact it alone. The current sitting President would have to sign it, or allow it to become law without a veto.

As for Members of Congress, rather than impeachment, the Constitution provides the remedy of “expulsion” in Article I, section 5: “Each House may determine the rules of its proceedings, punish its Members for disorderly behavior, and with the concurrence of two-thirds, expel a Member.” Only one chamber determines the fate of its own Members, with a 2/3 vote. Concurrence of the other body is not needed, so there is no trial.

Expulsions have been rare in congressional history because Members prefer to let constituents decide whether or not to remove a sitting Member by simply not re-electing him/her. In the Senate, 15 Senators have been expelled: 1 in 1797 and 14 for treason during the Civil War. In the House, 4 Representatives have been expelled - 3 for treason during the Civil War and 1 [Rep. Ozzie Meyers, D-PA] in 1980 for bribery [cash transfer caught on videotape by the FBI].

Some states have enacted “recall” election laws aimed at state level officials and legislators. Court rulings have established that Members of Congress, as federal constitutional officers, are not subject to recall under the authority of a state law. Nor is a recall special election procedure provided for in the U.S. Constitution for federal officials. Recall remains a state remedy. The removal of Members of the U.S. Congress is left to the expulsion clause of the Constitution, or to the wish of voters to hold them accountable for their actions by refusing to re-elect them in the next regular election.