As many as 45 Senate Republicans backed a failed effort on Tuesday to halt Donald Trump's impeachment trial, in a sign of party unity and the former president's continued sway over the GOP.

It also makes clear a conviction of Trump for "incitement of insurrection" after the deadly Capitol siege on January 6 is unlikely.

Republican Senator Rand Paul made a motion on the Senate floor that would have required the chamber to vote on whether Trump's trial in February violates the US Constitution.

The Democratic-led Senate blocked the motion in a 55-45 vote.

The vote means the trial on Trump's impeachment will begin as scheduled the week of February 8. The House impeached him January 13, just a week after the deadly insurrection in which five people died.

'Loss is actually a victory'

But only five Republican lawmakers joined Democrats to reject Paul's motion, far short of the 17 Republicans who would need to vote to convict Trump on the impeachment charge.

"It's one of the few times in Washington where a loss is actually a victory," Paul later told reporters. "Forty-five votes means the impeachment trial is dead on arrival."

Paul and other Republicans contend that the proceedings are unconstitutional because Trump left office last Wednesday and the trial will be overseen by Democratic Senator Patrick Leahy instead of by US Chief Justice John Roberts.

Leahy, 80, was hospitalized for observation on Tuesday evening after not feeling well, his spokesman David Carle said in a statement, which did not provide further details.

Some Republican senators who backed Paul's motion said their vote on Tuesday did not indicate how they might come down on Trump's guilt or innocence after a trial.

"It's a totally different issue as far as I'm concerned," Republican Senator Rob Portman told reporters.

'Flat-out wrong'

There is a debate among scholars over whether the Senate can hold a trial for Trump now that he has left office.

Democratic Senate Majority Leader Chuck Schumer, who moved to thwart Paul's motion, dismissed the Republican constitutional claim as "flat-out wrong" and said it would provide "a constitutional get-out-of-jail-free card" for presidents guilty of misconduct.

The nine House Democrats prosecuting the case against Trump carried the sole impeachment charge across the Capitol on Monday evening in a solemn and ceremonial march along the same halls the rioters ransacked three weeks ago.