By Susan Cornwell, Patricia Zengerle and Susan Heavey

WASHINGTON (Reuters) – 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.

Senators will begin the first of two planned 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, set to begin at about 1 p.m. (1800 GMT), precedes a 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 but allowing witnesses such as Bolton could inflict political damage on the Republican president as he seeks re-election on Nov.3.

Removing Trump 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.

Trump lashed out at Bolton on Twitter. He said Bolton “couldn’t get approved for the Ambassador to the U.N. years ago, couldn’t get approved for anything since, ‘begged’ me for a non Senate approved job” and added that “if I listened to him, we would be in World War Six by now.”

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?”

Contradicting Trump’s version of events, Bolton wrote in an unpublished book manuscript that the president told him he wanted to freeze $391 million in security aid to Ukraine until Kiev pursued investigations into Democrats,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 the impeachment charges. 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.

Biden is a leading candidate for the Democratic nomination to face Trump in November.

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.

Republican Senator Lindsey Graham, a Trump ally, remained opposed to witnesses but said, “I am concerned when John Bolton’s credibility is attacked, it makes it more likely some will feel the need to call him as a witness. In that event, it would be important for the president and his team to call witnesses on other issues.”

PARNAS ARRIVES

Ukrainian-born U.S. businessman Lev Parnas, who worked with Trump’s personal lawyer Rudy Giuliani to press Ukraine to pursue the politically motivated investigations at the center of the impeachment drama, arrived in Washington and headed toward the U.S. Capitol, surrounded by reporters.

Parnas will not be able to enter the Senate gallery where the trial is taking place because he is wearing a court-ordered ankle-monitoring device, his lawyer said. Parnas, facing federal campaign finance charges in New York relating to donations to a pro-Trump political group and others, has provided information to House Democrats damaging to Trump.

Parnas indicated he would be willing to testify in the trial, adding, “The president knew everything that was going on with Ukraine.”

Wednesday’s 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.

Trump’s legal team wrapped up its opening arguments in the trial on Tuesday, saying he president did not commit any impeachable offenses even if what Bolton said was true.

The Senate is expected on Friday to debate and vote on whether to call witnesses. Top Senate Democrat Chuck Schumer made a fresh pitch for Republican support for witnesses, saying calling them would not result in a lengthy delay in the trial.

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.

The younger 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.”

Schumer told reporters that Republicans have a Senate majority and could vote to call Hunter Biden if they want to, but added, “Hunter Biden is irrelevant and a distraction.”

(Reporting by Richard Cowan, David Morgan, Doina Chiacu, Patricia Zengerle, Susan Heavey, Susan Cornwell, Makini Brice, Karen Freifeld and Lisa Lambert; Writing by Will Dunham; Editing by Howard Goller and Alistair Bell)