DONALD TRUMP’S DEFENCE has said a trade of US military aid for political favours — even if proven — can not be grounds for his impeachment.

The claim, coming as the US president’s Senate trial continued yesterday, marked a striking shift from Trump’s claims of “perfect” dealings with Ukraine.

Trump’s defenders relied on retired professor Alan Dershowitz, a member of their team, who told senators every politician conflated his own interest with the public interest.

“It cannot be impeachable,” he declared.

Democrats are pressing to force the Senate to call more witnesses to testify, but Republicans appear intent on bringing the impeachment trial to a vote of acquittal, possibly in a matter of days.

Even new revelations from former national security adviser John Bolton are being countered by the president’s lawyers, who used yesterday’s unusual question-and-answer session to warn off prolonging the proceedings, insisting senators had heard enough.

Democrats argued Bolton’s forthcoming book can not be ignored.

It contends he personally heard Trump say he wanted military aid withheld from Ukraine until it agreed to investigate Joe Biden and his son, Hunter Biden — the abuse of power charge that is the first article of impeachment.

As Chief Justice John Roberts fielded queries, Texas Republican Ted Cruz asked if it mattered whether there was a quid pro quo in such actions as those conducted by Trump.

Dershowitz said it did not, adding many politicians equated their re-election with the public good.

“That’s why it’s so dangerous to try to psychoanalyse a president,” he said.

Representative Adam Schiff, the Democrat leading the House prosecutors, appeared stunned.

“All quid pro quos are not the same,” he retorted, saying some might be acceptable and some not. “And you don’t need to be a mind reader to figure out which is which. For one thing, you can ask John Bolton.”

With the vote on calling witnesses expected by Friday, Democrats are making a last-ditch push to sway Republicans to call Bolton and others to appear for testimony and ensure a “fair trial”.

Trump faces charges from the House that he abused his power like no other president, jeopardising Ukraine, and US-Ukraine relations, by using the military aid as leverage while the vulnerable ally battled Russia.

The second article of impeachment says Trump then obstructed the House probe in a way that threatened the nation’s three-branch system of checks and balances.

Over two days, senators are grilling the House Democrats prosecuting the case and the Republican president’s defence team.

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Dozens of questions were asked and answered yesterday in rapid-fire fashion, with senators under orders to sit silently without comment, submitting their questions in writing. They expect to continue today.

Democratic Leader Chuck Schumer asked whether the Senate could render a fair verdict without hearing from Bolton or acting White House Chief of Staff Mick Mulvaney, both potential eyewitnesses to Trump’s actions.

“Don’t wait for the book. Don’t wait till 17 March, when it is in black and white to find out the answer to your question,” Schiff told the Senate.

That publication date is now in doubt. The White House yesterday released a letter to Bolton’s attorney objecting to “significant amounts of classified information” in the manuscript, including at the top secret level.

Bolton and his lawyer have insisted the book does not contain any classified information.

The White House action could delay the book’s publication if Bolton, who resigned last September — Trump says he was fired — is forced to revise his draft.

GOP senators are straining to balance the new revelations with pressure for quick acquittal. They have been sternly warned by party leaders that calling Bolton as a witness could entangle the trial in lengthy legal battles and delay Trump’s expected acquittal.

Most Republican senators don’t want to extend the trial by calling Mr Bolton, and most Democrats would rather avoid dragging the Bidens further into the impeachment proceedings. The Bidens were a focus of defence arguments, though no evidence of wrongdoing has emerged.