The Senate Republican leader, Mitch McConnell, said Donald Trump was “crazy” and vowed never to speak to him again after the Capitol attack – then voted both to call Trump’s impeachment unconstitutional and to acquit the former president in his second Senate trial.

McConnell’s deliberations are reported in a forthcoming book, Unchecked: The Untold Story Behind Congress’s Botched Impeachments of Donald Trump, by Rachael Bade of Politico and Karoun Demirjian of the Washington Post. An extract was published on Wednesday.

According to Bade and Demirjian, on 6 January 2021, after the deadly attack on Congress by Trump supporters seeking to stop certification of Joe Biden’s election win, McConnell spoke to staffers in his Capitol office.

“We’ve all known that Trump is crazy,” he said. “I’m done with him. I will never speak to him again.”

But, the authors add, “while McConnell was ready to be done with Trump, his party, it seemed, was not. To his chagrin, a large chunk of his members were once again coalescing around the former president. And they were about to put him in a bind.”

Twenty days later, McConnell agonised over what he “knew would be one of the most pivotal votes of his career”.

The vote was forced by Rand Paul, a senator from Kentucky, in an attempt to declare Trump’s impeachment over the Capitol attack unconstitutional, given that he had then left office.

The authors report that McConnell and an aide argued about the issue. But though the Senate leader wasn’t convinced by Paul’s argument, he “had never led such a rebellion” against another Republican and “wasn’t sure he was up to the task”.

McConnell voted to declare the impeachment unconstitutional. When Trump went to trial, Bade and Demirjian say, McConnell considered voting to convict. But he voted to acquit and only excoriated Trump in a speech on the Senate floor after the acquittal was confirmed.

McConnell’s view that Trump was finished after the Capitol attack has been reported elsewhere. In their book This Will Not Pass Jonathan Martin and Alexander Burns report that McConnell was “exhilarated” and told staffers Trump was a “despicable human being” he would fight politically.

Burns and Martin also report that McConnell asked one of them about discussions of the 25th amendment, the constitutional process to remove a president incapable of the office.

“He put a gun to his head and pulled the trigger,” they quote McConnell as saying. “Couldn’t have happened at a better time.”

Burns and Martin also say McConnell believed he would regain control of his party, saying: “We crushed the sons of bitches [before] and that’s what we’re going to do in the primary in ’22.”

That has not proved the case.

Bade and Demirjian report that after the Capitol attack, McConnell consulted extensively with Liz Cheney, the Wyoming congresswoman who emerged as a figurehead for anti-Trump Republicans.

Worried that Trump would use the Capitol attack to fuel another White House run, McConnell reportedly told Cheney Republicans should “just ignore him”.

In August this year, Cheney lost her primary to a Trump-backed challenger. Other anti-Trump Republicans have met the same fate – or retired.

Trump continues to abuse and attack McConnell, seeking his replacement.