WASHINGTON (Reuters) - President Donald Trump’s fellow Republicans in the U.S. Senate came under renewed pressure on Monday to allow witnesses in his

impeachment trial, while his defense team largely ignored disclosures from a former White House adviser.

The elephant in the room on Day 2 of Trump’s defense arguments was John Bolton, the former national security adviser whose unpublished book manuscript, according to the New York Times, included disclosures that go to the heart of the abuse of power charge against Trump.

Bolton wrote that Trump told him he wanted to freeze $391 million in security aid to Ukraine until Kiev helped with investigations into Democrats, including political rival Joe Biden, and his son Hunter Biden, the Times reported.

The Bolton disclosures prompted new calls by Democrats for Bolton and other witnesses to testify. Trump is accused of abusing the power of his office in seeking foreign interference in a U.S. election and of obstructing Congress.

Trump’s lawyers said after about seven hours of arguments that they would resume their presentation on Tuesday.

Republican Senator Mitt Romney, a moderate who has at times criticized Trump, said there was a growing likelihood that at least four Republican senators would choose to call Bolton to testify, which would give Democrats the votes necessary in the Republican-led Senate to summon him.

The Senate may resolve the issue of whether to call witnesses in a vote on Friday or Saturday. Democrats said the Bolton manuscript made it all the more pressing for the Senate to call Bolton as a witness.

The Democratic-led House of Representatives impeached Trump last month, setting up the trial in the Senate on whether he should be removed from office. Trump is expected to be acquitted in the 100-seat chamber, where Republicans hold 53 seats.

The White House directed current and former administration officials not to provide testimony or documents in the House inquiry that preceded the trial, and Senate Republicans have so far refused to allow any witnesses or new evidence.

Senator Ted Cruz, a staunch Trump defender, said the Bolton book would not “impact the legal issue before this Senate.”

Trump’s legal team on Monday resumed its presentation of opening arguments in the trial, including remarks by Ken Starr, the former independent counsel whose investigation into a sex scandal led to the 1998 impeachment of President Bill Clinton, a Democrat. Another Trump lawyer, Jane Raskin, defended his personal attorney Rudy Giuliani.

Bolton was barely mentioned. In an apparent reference to the manuscript leak, Trump lawyer Jay Sekulow said: “We deal with publicly available information. We do not deal with speculation, allegations that are not based on evidentiary standards at all.”

ALL ABOUT BIDEN

Instead, defense lawyers turned to Joe Biden, one of Trump’s leading Democratic rivals as he seeks re-election in November, and Hunter Biden, who sat on the board of Ukrainian energy company Burisma while his father was U.S. vice president.

Attorney Pam Bondi defended Trump’s use of unsupported corruption allegations against the Bidens as the basis for his demand that Ukraine investigate them.

She presented a series of media reports, Ukrainian gas company records and excerpts from impeachment inquiry testimony in an attempt to demonstrate that a range of independent observers were concerned that Hunter Biden’s role posed a potential conflict of interest.

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“All we are saying is that there was a basis to talk about this, to raise this issue, and that is enough,” Bondi said.

Ukrainian officials have said they found no indication that Hunter Biden had broken any law. Biden campaign spokesman Andrew Bates on Monday dismissed Bondi’s allegations, saying they had been widely discredited.

“The president’s lawyers spent about two hours trashing the Bidens,” said Representative Adam Schiff, the lead House impeachment prosecutor. He added that Trump’s defense team could not explain why Trump took an interest in corruption and Burisma only when Biden became a presidential candidate.

Republican Senator Joni Ernst of Iowa said it was appropriate to bring up the Bidens, and looked to next Monday’s presidential caucuses in her home state. “Will they be supporting Vice President Biden at this point? Not sure about that,” she said during a break in the trial.

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Some of Trump’s strongest Republican backers in Congress have threatened to make the impeachment trial about the Bidens.

Biden himself predicted the attacks, warning voters at a campaign event in Ankeny, Iowa, over the weekend: Turn it on Monday, watch the news. It’s going to be all about Biden.”

AID TO UKRAINE

Democrats have said Trump used the aid to a vulnerable ally facing Russian aggression as leverage to get a foreign country to help him smear a domestic political rival.

Trump denied telling Bolton that he sought to use the aid to pressure Ukrainian President Volodymyr Zelenskiy to investigate the Bidens on unsubstantiated corruption allegations.

“I haven’t seen the manuscript, but I can tell you nothing was ever said to John Bolton,” Trump told reporters.

But moderate Republican Senator Susan Collins said the reports regarding Bolton’s book “strengthen the case for witnesses.”

“Every few days something comes out, and Americans are saying they want witnesses & documents,” Senate Democratic leader Chuck Schumer said on Twitter

Starr, who himself recommended Clinton’s impeachment after investigating the former president’s sexual relationship with a White House intern, called impeachment an overused tool.

“Go to court. It really is as simple as that, I don’t need to belabor the point,” Starr said.

This is only the third presidential impeachment trial in U.S. history.

Another Clinton impeachment alumnus, Alan Dershowitz, carried a pair of worn antique books as he began closing the day’s defense with an argument disputed by most legal scholars. “Purely non-criminal conduct, including abuse of power and obstruction of Congress, are outside the range of impeachable offenses,” he said.

Dershowitz himself argued the opposite view during Clinton’s impeachment.