Republican lawmakers demonstrated Sunday that Democrats will have a fight on their hands to convict Donald Trump when the Senate next month opens its first-ever impeachment trial of a former president.

Speaker Nancy Pelosi is expected on Monday to send senators a single article of impeachment passed in the House of Representatives that blames Trump for inciting the chaotic Capitol invasion of January 6, which left five people dead.

But as both sides prepared for what is expected to be a relatively quick trial, senior Republicans pushed back with both political and constitutional arguments, raising doubts that Democrats, who control 50 seats in the 100-seat chamber, can secure the 17 Republican votes needed to convict.

"I think the trial is stupid. I think it's counterproductive. We already have a flaming fire in this country and it's like taking a bunch of gasoline and pouring it on top of the fire," Marco Rubio, the top Republican on the Senate Intelligence Committee, told "Fox News Sunday".

He acknowledged that Trump — who had urged thousands of his supporters to flock to Washington and protest the congressional certification of Biden's victory — "bears some responsibility for what happened".

But to "stir it up again" would be bad for the country, said Rubio, a presidential candidate beaten by Trump in the 2016 primary.

Constitutional question 

Other Republicans argued that the Senate has no authority to put a private citizen — as Trump now is — on trial.

Senator Mike Rounds told NBC's "Meet the Press" that the constitution does not allow for the impeachment of a former president.

"There are other things we'd rather be working on," including confirming more of Biden's Cabinet nominees, he said.

But Senator Mitt Romney, the Republicans’ 2012 presidential candidate and a frequent Trump critic, told CNN that "the preponderance of legal opinion is that an impeachment trial after a president has left office is constitutional. I believe that’s the case”.

The Utah Republican — the only member of his party to vote to convict Trump in his first impeachment trial — hinted that he may be leaning the same way now.

He said he believed "that what is being alleged and what we saw, which is incitement to insurrection, is an impeachable offence. If not, what is?”

Hands-off approach 

Biden himself has publicly taken a hands-off approach, eager to put Trump in the rear-view mirror and seek quick progress on fighting the coronavirus pandemic and reviving a devastated economy.

Biden spokeswoman Jen Psaki has said the president "believes that it’s up to the Senate and Congress to determine how they will hold the former president accountable”.

As Democrats worked to prepare the case against Trump, one of those who will present it in the Senate — Representative Madeleine Dean — said she hoped it would move quickly.

"I would expect it would go faster” than the 2020 impeachment trial, which lasted 21 days, she told CNN.

Dean said she was in the House chamber during the "terrifying moment” when the invading mob began pounding on its doors, chanting: "Hang Vice President Pence.”

She said Democrats would demand accountability for Trump’s role.

"It is an extraordinarily heinous presidential crime,” she said.