(Corrects spelling of Ukraine president Zelenskiy's first name in paragraph 8, Volodymyr instead of Volodomyr)

By Richard Cowan and Susan Cornwell

WASHINGTON, Jan 27 (Reuters) - President Donald Trump's fellow Republicans in the U.S. Senate came under fresh pressure on Monday to allow witnesses in his impeachment trial, while his defense team took aim at Democrats' accusations that he mounted a pressure campaign against Ukraine for his own political ends.

A New York Times report that former national security adviser John Bolton has written in an unpublished book manuscript that Trump told him he wanted to freeze security aid to Ukraine until Kiev helped with politically beneficial investigations prompted fresh calls by Democrats for Bolton and other witnesses to testify at the trial.

Republican Senator Mitt Romney, a sometime critic of Trump, said there was a growing likelihood that at least four Republican senators would choose to call for Bolton to testify, which would give Democrats the votes necessary to summon him.

Senate Republicans so far have refused to allow any witnesses or new evidence in the trial that will determine whether Trump is removed from office.

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 paved the way for the impeachment of former President Bill Clinton, a Democrat, in 1998. Another Trump lawyer, Jane Raskin, also made a defense of his personal attorney Rudy Giuliani.

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

Democrats have said Trump used the aid - approved by the U.S. Congress to help Ukraine combat Russia-backed separatists - as leverage to get a foreign country to help him smear a domestic political rival. Biden is a leading contender for the Democratic nomination to face Trump in the Nov. 3 presidential election.

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. Hunter Biden worked for a Ukrainian energy firm while his father was U.S. vice president.

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Romney told reporters.

Another moderate Republican senator, Susan Collins, said the reports regarding Bolton's book "strengthen the case for witnesses." Senate Judiciary Committee Chairman Lindsey Graham, a Republican ally of Trump, said he would support issuing a subpoena to obtain Bolton's manuscript, a CNN reporter said on Twitter.

The Democratic-led House of Representatives impeached Trump last month on charges of abuse of power in his dealings with Ukraine and obstruction of Congress, setting up the trial in the Republican-led Senate. Trump is expected to be acquitted in the 100-seat Senate, where Republicans hold 53 seats and a two-thirds vote is required to convict and remove a president from office.

Trump denied telling Bolton he was seeking something in return for unfreezing the Ukrainian aid, which eventually was provided in September after the controversy became public.

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

DEFENSE ARGUMENT

In the initial hours of its second day of opening arguments, Trump's defense team did not mention Bolton. Lawyers instead took aim at House accusations that Trump's treatment of Zelenskiy amounted to a pressure campaign.

Defense lawyer Mike Purpura cited the Ukrainian president as saying he had felt no pressure, and said the security assistance was released anyway without the investigations taking place.

The defense pointed to testimony from witnesses in the House impeachment inquiry who said Trump's administration gave Ukraine more military help than Democratic President Barack Obama's prior administration.

"All of this ... demonstrates that there was no connection between security assistance and investigations," Purpura said.

Jay Sekulow, another Trump lawyer, added, "We live in a constitutional republic where you have deep policy concerns and deep differences. That should not be the basis of an impeachment."

Democrats prosecuting Trump in the trial have said Giuliani played a central and improper role in pressing Ukraine to carry out politically motivated investigations.

"The House managers would have you believe that Mr. Giuliani is at the center of this controversy. They've anointed him the proxy villain of the tale, the leader of a rogue operation," Raskin said.

"Their presentations were filled with ad hominem attacks and name calling: cold-blooded political operative, political bag man. But I suggest to you that he's front and center in their narrative for one reason and one reason alone: to distract from the fact that the evidence does not support their claims," Raskin added.

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.

"It completely blasts another hole in the president's defense," said Representative Adam Schiff, the head of the House Democratic team of "managers" prosecuting the case against Trump, told CNN.

Many Republicans want a speedy trial without witnesses or any evidence beyond the material amassed in the House impeachment inquiry.

The White House directed current and former administration officials not to provide testimony or documents in the House inquiry. Starr cited ways short of impeachment for the House to force a president to comply with its oversight obligations.

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

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.

"The Senate is being called to sit as the high court of impeachment all too frequently," Starr said. "How did we get here with presidential impeachment invoked in its inherently destabilizing and acrimonious way?"

This is only the third impeachment trial in American history.

(Additional Reporting by Pete Schroeder, Arshad Mohammed, Tim Ahmann, Makini Brice, Steve Holland, Jeff Mason and Lisa Lambert; Writing Alistair Bell; Editing by Will Dunham Editing by Andy Sullivan)

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