WASHINGTON (Reuters) - The Senate impeachment trial of Donald Trump on a charge of inciting the deadly Capitol siege could draw its first challenge on Tuesday, with a Republican senator arguing that trying a former president would violate the U.S. Constitution.

Trump is the only president to have been impeached by the House of Representatives twice and is the first to face a trial after leaving power, with the possibility of being disqualified from future public office if convicted by chamber's 100 senators serving as jurors. The trial is expected to begin on Feb. 9.

Senator Rand Paul, a Trump ally, has pledged to force a vote on whether the Constitution allows the Senate to try the former president who is now a private citizen. Trump left office on Jan. 20.

The House approved a single article of impeachment - the equivalent of an indictment in a criminal trial - on Jan. 13, accusing him of inciting an insurrection with an incendiary speech to supporters before they stormed the Capitol on Jan. 6. A police officer and four others died in the melee.

"I will force a vote on whether the Senate can hold a trial of a private citizen," Paul wrote on Twitter late on Monday, voicing an objection also raised by other Republicans.

Paul is expected to speak on the Senate floor around midday.

There is a debate among scholars over whether the Senate can hold a trial for Trump now that he has left office. Many experts have said "late impeachment" is constitutional, arguing that presidents who engage in misconduct late in their terms should not be immune from the very process set out in the Constitution for holding them accountable.

The Constitution makes clear that impeachment proceedings can result in disqualification from holding office the future, so there is still an active issue for the Senate to resolve, these scholars have said.

At least 17 Republican would need to join all 50 Democrats in the evenly divided Senate for Trump to be convicted, a two-thirds threshold that appears unlikely to be reached. Trump remains a powerful force among Republicans and his supporters have vowed to mount election challenges to lawmakers in the party who support conviction.

Some Republicans have criticized Trump's false claims of voting fraud and his failed efforts to overturn President Joe Biden's Nov. 3 election victory. But no Senate Republicans have said definitively that they plan to vote to convict him.

A vote on Paul's objection could come on Tuesday afternoon, when the 100 senators are expected to be sworn in for their role as jurors. Paul's office was not immediately available for comment.

Paul also has objected to plans to have Democratic Senator Patrick Leahy preside at the trial. U.S. Chief Justice John Roberts presided at Trump's first impeachment trial, which ended in his acquittal in February 2020 on charges of abuse of power and obstruction of Congress arising from his request that Ukraine investigate Biden and his son.

Paul cited language in the Constitution that states: "When the President of the United States is tried, the Chief Justice shall preside."

Although the Constitution calls on the chief justice to preside over presidential impeachment trials, a senator presides when the impeached is not the current president, a Senate source said. First elected to the chamber in 1974, Leahy, 80, holds the title of Senate president pro tempore.

The nine House Democrats who will serve as prosecutors set the trial in motion on Monday by delivering the article of impeachment to the Senate.

 

(Reporting by David Morgan, additional reporting by Jan Wolfe in Boston; Editing by Will Dunham and Scott Malone)