First Up at Trump’s Impeachment: Can a Former President Stand Trial?

The first issue that will be debated in the opening hours of former President Donald J. Trump’s impeachment trial on Tuesday will be the question of whether it is constitutional to put an impeached former president on trial.

The politics around the question are significant. Republicans have argued that the proceeding is unconstitutional, and by doing so, avoided addressing whether Mr. Trump committed impeachable offenses in his role in the Capitol riot.

Senate Republicans who voted last month to dismiss the trial as unconstitutional came under pressure on Sunday to re-evaluate their position when a leading conservative constitutional lawyer, Charles J. Cooper — who has been a close ally and adviser to Republican senators like Ted Cruz of Texas — argued in a Wall Street Journal op-ed that their claims about the constitutionality of the proceeding were unfounded.

The Senate has set aside four hours to debate this issue on Tuesday. Here’s what you need to know.

What have Republicans said?

House Democrats, who were joined by 10 Republicans, voted to impeach Mr. Trump a week before he left office for “incitement of insurrection.”

The impeachment put pressure on Senate Republicans to either condone or repudiate Mr. Trump’s conduct. Some set aside the question to instead focus on the process itself, arguing that whether or not Mr. Trump’s actions constituted high crimes and misdemeanors, the Senate could not try him because the Constitution does not allow a former president to stand trial for impeachment.

Two weeks ago, Senator Rand Paul of Kentucky forced a vote on the issue, moving to challenge the trial as unconstitutional. All but five Republicans sided with him. Democrats and constitutional scholars responded by arguing that the Republicans were simply clinging to a politically expedient argument to avoid crossing Mr. Trump, who remains popular among Republican voters.

What will Democrats argue?

The Democratic House impeachment managers are expected to broadly assert that a president can be put on trial for offenses committed in office, no matter when the trial is held. Otherwise, the Democrats say, there would be no way to hold to account a president who commits wrongdoing in the final weeks of a term.

“The framers of the Constitution feared a president who would corrupt his office by sparing ‘no efforts or means whatever to get himself re-elected,’” the nine impeachment managers wrote in a filing last week. “If provoking an insurrectionary riot against a joint session of Congress after losing an election is not an impeachable offense, it is hard to imagine what would be.”

Mr. Trump’s lawyers will most likely make the narrower and more technical argument that the Constitution forbids a former president to be put on trial.

“The Senate of the United States lacks jurisdiction over the 45th president because he holds no public office from which he can be removed, rendering the article of impeachment moot,” Mr. Trump’s lawyers, Bruce L. Castor Jr. and David Schoen, wrote in a 14-page response to the House managers last week.

What about the conservative lawyer?

In the opinion piece, Mr. Cooper took on the Republicans’ assertion that because the penalty for an impeachment conviction is removal from office, it was never intended to apply to a former president.

Mr. Cooper argued that the Constitution gives the Senate the power to bar convicted officials from holding office again. “It defies logic to suggest that the Senate is prohibited from trying and convicting former officeholders,” he wrote.

Mr. Cooper said that since Republicans voted on the issue last month, legal scholarship has evolved and “exposed the serious weakness” of their argument.

“The senators who supported Mr. Paul’s motion,” he wrote, “should reconsider their view and judge the former president’s misconduct on the merits.”

Mr. Cooper’s career has been largely focused on furthering the conservative constitutional legal movement. As a top Justice Department official in the Reagan administration, he wrote an opinion on whether employers could decline to hire someone who may have AIDS, which was criticized as discriminatory. As a private lawyer he has represented the National Rifle Association, advocates of prayer in school and defenders of the California same-sex marriage ban.

Senators will vote on the question.

Democrats seized on Mr. Cooper’s piece to bolster their case against Mr. Trump on Monday.

“That’s no liberal, that’s Chuck Cooper — a lawyer who represented House Republicans in a lawsuit against Speaker Pelosi, a former adviser to Senator Cruz’s presidential campaign — driving a stake into the central argument we’re going to hear from the former president’s counsel,” Senator Chuck Schumer of New York, the Democratic leader, in a speech on the Senate floor.

The impact of Mr. Cooper’s opinion article will be measured on Tuesday when Republicans are asked to vote on the question again. Some senators privately said that they were caught off guard by the vote last month on the issue after Mr. Paul raised it and have said that they are open to debating it and considering the constitutional question. Others — like Rob Portman of Ohio and Bill Cassidy of Louisiana — said they voted against the measure because they wanted to force a larger debate on the issue.

Mr. Cooper’s conservative credentials and decision to publicly lay out his argument will most likely push some senators to at least consider his view, according to Steven Teles, the author of “Never Trump: The Revolt of the Conservative Elites” and a professor at Johns Hopkins University.

“It does make it harder for them if Chuck Cooper is out there saying, ‘Come on,’” said Mr. Teles, who is also a senior fellow at the think tank Niskanen Center in Washington. “Everyone would like to dodge every difficult question with making a procedural argument. It’s harder to dodge that if a conservative legal authority is out there saying actually you can’t avoid the question.”