1.) Can a former U.S. president be impeached? Can he be convicted?
Those are two different constitutional questions. And U.S. President Donald Trump, impeached last week while still in office and—

2.) — potentially subject to conviction after departing, has obvious reason to offer a firm “no” to the second question.

Under the Constitution, the House of Representatives is authorized to impeach a president, and then the Senate is authorized to convict him. But that doesn’t

3.) — answer the questions about a former president.

Let’s start with the text. Article I says this: “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—

4.) — United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.”

5.) Article II says this: “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.”

6.) Under these provisions, it should be clear that the House cannot impeach someone who has never held public office, and who merely aspires to do so.
Suppose, for example, that a presidential candidate has done horrible things while, say, serving as governor of a state.—

7.) — The text makes it clear that impeachment is unavailable.

What about an ex-president? Could the House, now or next year, impeach Jimmy Carter? Bill Clinton? George W. Bush?
The Constitution doesn’t explicitly answer question, but there’s a strong argument best answer is, No

8.) The overriding goal of impeachment is to start a process that can culminate in removal from office. If a president is no longer in office, he can’t be removed. Main point of the impeachment clauses is to allow “We the People” to take action, in real time, against officials—

9.) — who are egregiously abusing their authority. If a president is a private citizen, that purpose of the clause no longer appears relevant.

There is a counterargument, which is that Article I refers not only to removal, but also to “disqualification to hold and enjoy any—

10.) — Office of honor, Trust or Profit under the United States.”

Can the Senate proceed to convict him after he left on Jan. 20, the day of Joe Biden’s inauguration, and becomes a private citizen?

11.) You could easily argue that it can. In impeaching Trump, If the Senate proceeds to a trial, it is merely continuing the process. The disqualification provision makes that clear.
The counterargument is also not hard to sketch. Once Trump has left office, he cannot be removed—

12.) — Removal is the central goal of the impeachment provision.

The Constitution does not clearly resolve the central question; it contains a genuine gap. Someone has to fill it. The impeachment process has obvious political dimensions, and the Supreme Court is highly unlikely—

13.) — to get involved.

Which means, Senate is the right institution to fill the gap? Without offense to the Constitution, it could choose to stay its hand. Without offense to the Constitution, it could choose to complete the process that the House started But, —