Moderate Republican senators were under an intense spotlight on Wednesday as the impeachment trial of Donald Trump moved into a dramatic new phase – and the president lashed out at former national security adviser John Bolton, who Democrats want to call as a key witness.

Trump’s Twitter attack on Bolton – and subsequent moves by the White House to block his potentially damaging planned book – came amid signs the Senate majority leader, Mitch McConnell, does not have enough Republican votes locked down to block Democrats’ efforts force witnesses into the impeachment trial.

Leading Republican moderates Susan Collins, Mitt Romney and Lisa Murkowski kicked off the questioning phrase of the trial on Wednesday by asking, if Trump had multiple motives for delaying US military aid to Ukraine, how the Senate would handle that.

That goes to the heart of the impeachment accusation that Trump inappropriately pressured Ukraine by holding back aid until the Ukrainian government agreed to investigate the president’s leading election rival Joe Biden, in what Democrats charge is an abuse of power.

A looming question over Wednesday’s impeachment proceedings is whether centrist senators will side with Democrats to form a majority voting bloc to bring witnesses in.

While Democrats have Bolton at the top of their list, the Senate minority leader, Chuck Schumer, brushed off the prospect of moderate Democrat Joe Manchin of West Virginia supporting the prospect of Biden’s son Hunter being requested by Republicans to testify about his time working for a Ukrainian gas company.

Manchin said on MSNBC on Wednesday he thought Hunter Biden should testify, which most Democrats have opposed.

“We are totally united and have been totally united for a month and continue to be,” Schumer said of his Senate members at a press conference.

Meanwhile, Trump attacked Bolton after reports that a draft manuscript of Bolton’s forthcoming book claims Trump directly linked a delay in military aid to Ukraine to a condition that the Ukrainian government investigate his Democratic rivals, especially Joe Biden.

Trump wrote of Bolton: “… if I listened to him, we would be in World War Six by now, and goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?”

One of Trump’s defense arguments during the impeachment inquiry was that those two were not connected. Critics say he was trying to use a vulnerable Ukraine to help his re-election chances; Trump said he was just trying to root out corruption there.

....many more mistakes of judgement, gets fired because frankly, if I listened to him, we would be in World War Six by now, and goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this? — Donald J. Trump (@realDonaldTrump) January 29, 2020

Trump maintains Bolton was fired last fall, while Bolton claimed he resigned.

On Wednesday, Trump also complained about Bolton’s timing. He wrote: “Why didn’t John Bolton complain about this “nonsense” a long time ago, when he was very publicly terminated. He said, not that it matters, NOTHING!”

The White House later sent a letter to Bolton’s attorney, which claimed his client’s book cannot be published in its current form because it contains top secret material.

McConnell is under pressure to round up his caucus and move to a vote by the end of the week first to block witnesses and then to acquit Trump on the impeachment charges altogether.

McConnell told his caucus in a meeting on Tuesday night, according to multiple reports, of the stakes.

With an unknown number of Republican senators still undecided about calling witnesses, McConnell could still get the votes he needs to block witnesses and stop the trial from reeling off into unpredictable – and potentially hazardous – territory for the president. At least four Republicans would need to join Democrats to force witness testimony.

Murkowski met with McConnell on Wednesday morning, adding to speculation that she could join Democrats in supporting a motion to bring in witnesses, according to a Senate aide.

Trump’s defense team and his Republican allies have argued vehemently against the inclusion of witnesses at the trial, saying they already had enough information to decide the case and that the Senate should not be burdened by what they have framed as an incomplete process in the House of Representatives.

But those arguments appear not to have been persuasive to the necessary number of senators. Trump’s lawyers concluded their opening arguments on Tuesday.

Led by Trump’s personal lawyer Jay Sekulow, the defense team dismissed objections to Trump’s conduct towards Ukraine as “policy disagreements” and warned senators not to “lower the bar of impeachment” by voting to convict the president.

A furious Trump complained on Twitter on Wednesday morning that Democrats would never be satisfied. During the inquiry process in the House of Representatives last year, the White House blocked senior administration officials from testifying in the House.

No matter how many witnesses you give the Democrats, no matter how much information is given, like the quickly produced Transcripts, it will NEVER be enough for them. They will always scream UNFAIR. The Impeachment Hoax is just another political CON JOB! — Donald J. Trump (@realDonaldTrump) January 29, 2020

No witnesses have appeared at the trial in the Senate, with House managers detailing the case for the prosecution of Trump that he abused the power of his office and, in gagging officials and blocking the release of some documentary evidence, obstructed Congress – forming the two articles of impeachment relating to Trump’s conduct with regard to Ukraine.

Trump has no plans to appear at his own trial and has been pressing Congress to get it finished before he has to give his State of the Union address next Tuesday – the day after the crucial Iowa caucuses occur as the first voting event in the primary contest to choose the Democratic party nominee to challenge Trump for the White House this November.

Moderate Biden is jostling for the top spot in opinion polls with resurgent leftwinger Bernie Sanders.