Donald Trump on Wednesday blasted his former national security advisor John Bolton as he emerged as a potential 11th hour witness in president's impeachment trial, complicating the White House's bid for a speedy acquittal.

Senators were poised to question the president's accusers and his lawyers for the first time later in the day as a fight over Bolton's testimony played out behind the scenes. Democrats are pressing for the Senate to subpoena Bolton after reports that his coming White House tell-all book corroborates the abuse-of-power impeachment charge against Trump.

Bolton reportedly writes that the president personally told him in August that a freeze in military aid to Ukraine was directly linked to Trump's demand that Kiev announce investigations into Joe Biden, the current frontrunner for the Democratic presidential nomination.

In an early-morning tweet, Trump said Bolton, whom he fired in September after 17 months as White House national security advisor, had shown poor judgement in a job he had begged for.

Bolton is "a guy who couldn't get approved for the Ambassador to the UN years ago, couldn't get approved for anything since," Trump wrote. Trump said he hired the veteran diplomat and prominent "hawk" against the advice of others. But Bolton "got fired because frankly, if I listened to him, we would be in World War Six by now."

"(He) goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?" Trump said. With the 100-member Senate serving as the jury in his trial, Trump called on Republicans to reject a push for witnesses to testify when the issue comes up on Friday. "Remember Republicans, the Democrats already had 17 witnesses, we were given NONE! Witnesses are up to the House, not up to the Senate. Don't let the Dems play you!" he wrote on Twitter.

The Senate trial, where Trump faces charges of abuse of power and obstruction of Congress in the Ukraine affair, resumes Wednesday afternoon in Washington with senators submitting questions to the House prosecutors and Trump's defense team.

Both sides spent the last week presenting their cases for and against Trump, who is accused of abuse of power and obstruction of Congress, as well as the constitutional underpinnings of the charges.

If convicted by the Senate, Trump would be removed from office. But a two-thirds majority is required for that and the Republicans' 53-47 Senate majority virtually assures Trump will be acquitted. Only two other presidents before have been put on trial in the Senate, and neither was convicted and removed. Before the explosive revelations about what Bolton knew, the White House had assumed the trial could be wrapped up with no witnesses and a solid acquittal by this weekend, just in time for Trump's annual State of the Union address before Congress on Tuesday, February 4.

Now Democrats hope that a minimum of four Republicans will side with them in calling for witnesses, with Bolton top on their list. The issue comes up for a vote Friday after the trial's questions phase is complete. "We have an obligation and a responsibility to have a thorough trial. How do you have a trial without witnesses and evidence?" said Democratic Senator Joe Manchin on MSNBC.

"If you are going to have a fair trial ... then we need relevant witnesses," said Hakeem Jeffries, one of the House impeachment managers who act as prosecutors in the trial. "And who is more relevant in this particular instance that John Bolton, who had a direct conversation with President Trump about President Trump's desire to exchange phony political investigations into his rival for the release of $319 million in military aid?"

Senate Republican leader Mitch McConnell was reported to be no longer confident he could prevent enough members of his party from crossing over to give Democrats the needed 51 votes for witnesses.

John Cornyn, a senior Republican senator, however, told MSNBC he thought his side would be able to block the move and take the trial to a final vote at the end of the week. "Evidence is undisputed. More witnesses are unnecessary and would allow House Managers to tie up the Senate for the indefinite future over protracted court fights over executive privilege," he said on Twitter.