U.S. President Donald Trump unleashed his sharpest attack yet on John Bolton after his former national security adviser depicted Trump as playing a central role in a politically motivated pressure campaign on Ukraine, as the U.S. Senate prepared on Wednesday to enter a new phase in the president's impeachment trial.

Trump lashed out at Bolton on Twitter. The president said Bolton "couldn't get approved for the Ambassador to the UN years ago, couldn't get approved for anything since, 'begged' me for a non Senate approved job" and "mistakenly says 'Libyan Model' on T.V. (in reference to how to handle North Korea) - adding that "if I listened to him, we would be in World War Six by now."

Trump appeared to be referencing Bolton's long reputation as a hawk and his past advocacy for the U.S. to bomb Iran. Trump added that Bolton, who left his White House post in September, "goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?"

Senators will begin up to two days of posing questions to both Trump's legal team and the Democrats in the House of Representatives lawmakers who have served as prosecutors in the trial on charges of abusing power and obstructing Congress arising from his request that Ukraine investigate political rival Joe Biden.

The questioning precedes a crucial vote later in the week on whether to call witnesses including Bolton, as Democrats have sought. Senate Republicans so far have refused to allow any witnesses or new evidence in the trial, with Republican leaders hoping to vote as quickly as possible to acquit Trump, leaving him in office.

Senate Majority Leader Mitch McConnell told senators on Tuesday he did not have the votes to block Democrats from calling witnesses because some Republicans remained uncommitted, several media outlets reported. Democrats need four Republican senators to join them in voting for witnesses in order to get a majority in the 100-seat Senate.

The Senate is expected to acquit Trump. Removing him from office would take a two-thirds majority. There are 53 Republican senators and none of them has publicly advocated removal. The Democratic-led House impeached Trump last month.

Contradicting Trump's version of events, Bolton wrote in an unpublished book manuscript that the president told him he wanted to freeze $391 million in security aid to Ukraine until Kiev pursued investigations into Democrats, including Biden and the former vice president's son Hunter Biden, the New York Times reported.

Bolton's allegations go to the heart of impeachment charges against Trump. Democrats have said Trump abused his power by using the security aid - passed by Congress to help Ukraine battle Russia-backed separatists - as leverage to get a foreign power to smear a political rival.

Trump is seeking re-election on Nov. 3. Biden is a leading candidate for the Democratic nomination to face Trump.

'TIME FOR THIS TO END'

Trump has denied telling Bolton he sought to use the aid as leverage to get Ukraine to investigate the Bidens. Trump has said he fired Bolton. Bolton, a foreign policy hawk who served as a temporary "recess appointee" as American ambassador to the United Nations under Republican former President George W. Bush, has said he quit.

The questions during the trial will alternate between Republican and Democratic senators. They will be submitted in writing and read aloud by U.S. Chief Justice John Roberts, who is presiding over the trial.

The questions will take up to eight hours a day over Wednesday and Thursday. There is no time limit on the answers, which cannot be challenged by senators, Senate aides said.

Trump's legal team wrapped up its third and final day of opening arguments in the trial on Tuesday, saying that the president did not commit any impeachable offenses even if what Bolton said was true.

Trump's legal team sought to minimize the importance of Bolton's allegations and called the manuscript "inadmissible."

"It is time for this to end, here and now," White House Counsel Pat Cipollone told the Senate.

The Senate is expected to turn on Friday to a debate and vote on whether to call witnesses in the trial. Republican senators met privately after Tuesday's trial session to discuss calling witnesses, but said afterward there was no resolution.

Trump urged Republicans to stand firm against witnesses, writing on Twitter: "Don't let the Dems play you!"

Senator Joe Manchin, a West Virginia Democrat whose state strongly backs Trump, said it was critical to have witnesses and that he had yet to decide whether to acquit Trump, saying the two days of questioning would help him decide.

In a break with his party, Manchin also said Hunter Biden, like Bolton, would be a relevant witness who should be called. Republicans oppose calling any witnesses but have said they would want to hear from Hunter Biden if witnesses are permitted.

Hunter Biden had worked for a Ukrainian energy company when his father was vice president. Trump has made unsubstantiated allegations of corruption against the Bidens.

In an interview with MSNBC, Manchin said that "being afraid to put up anybody who might have pertinent information is wrong, whether you're Democrat or Republican."