Pressure mounted on Republicans on Monday to call former national security advisor John Bolton as a witness at Donald Trump's Senate impeachment trial following explosive new revelations about the US president's dealings with Ukraine.
As Clinton impeachment investigator Ken Starr resumed the White House defense of Trump on the Senate floor, at least three Republican senators indicated they would favor hearing testimony from the 71-year-old Bolton.
According to The New York Times, Bolton, in a draft of his upcoming book, says that Trump told him in August that he wanted to freeze military aid to Ukraine until Kiev opened an investigation into his potential November election rival Joe Biden.
The allegation that Trump withheld the aid for his own political purposes is at the heart of Trump's impeachment by the Democratic-controlled House of Representatives for abuse of power and obstruction of Congress.
Four Republicans would need to join forces with Democrats in the Senate -- where Republicans hold a 53 to 47 seat edge -- to allow testimony from Bolton and other witnesses sought by Democrats.
Republican Senator Mitt Romney of Utah said he believed there may be enough Republican backing to compel Bolton to testify.
"It's increasingly apparent that it would be important to hear from John Bolton," Romney told reporters. "I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton."
Susan Collins, a Republican senator from Maine, said the Bolton reports "strengthen the case for witnesses."
Lisa Murkowski, a Republican from Alaska, said she has always been "curious as to what John Bolton might have to say."
Chief Democratic prosecutor Adam Schiff welcomed the news that Republicans were considering calling Bolton to testify at what is just the third impeachment trial of a president in US history.
"You can't have a trial, a meaningful trial, without witnesses and you certainly can't have one without John Bolton," Schiff said. "Senators should not wait until March 17 when the book comes out. They should demand this information while it would still be pertinent to their decision."
While just 51 senators are required to allow further trial witnesses, 67 senators -- a two-thirds majority -- are needed to remove Trump from office.
- 'Age of impeachment' -
Trump lashed out at Bolton on Twitter, denying he had ever told his former aide that military assistance to Ukraine was tied to Kiev investigating Biden or his son Hunter, who served on the board of a Ukrainian gas company.
"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump said.
"In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."
Speaking later to reporters, Trump said he had not yet seen the manuscript, which was submitted to the White House last month for security clearance.
The White House began its defense presentation on Monday with a historical overview of past impeachments from Starr, whose investigation led to the 1998 impeachment of president Bill Clinton.
"The Senate is being called to sit as the high court of impeachment all too frequently," Starr said. "Indeed we're living in what I think can be aptly described as the age of impeachment."
Starr argued that efforts to remove Trump from office were unconstitutional because unlike in the cases of Clinton and former presidents Richard Nixon the articles of impeachment did not allege any actual crime by the president.
"Like war, impeachment is hell," Starr told the 100 senators who will decide Trump's fate. "At least presidential impeachment is hell."
He also said in the Nixon and Clinton cases there was a bipartisan consensus between Democrats and Republicans in the House to impeach the president.
"Not a single House member of the president's party supported either of the two articles of impeachment," Starr said. "Not one."
"Impeachment must be bipartisan in nature," he argued.
- Executive privilege -
Starr also reprised the argument made by White House lawyers on Saturday that American voters -- not Congress -- should decide in November whether Trump should be removed from office.
Starr's appearance drew a caustic comment from former Democratic senator Claire McCaskill, among others.
"I'm scratching my head here," McCaskill said. "They get the lawyer who thought an extramarital affair was impeachable to argue that withholding urgent military assistance to an ally in war against our enemy for a political favor is not impeachable.
"If it wasn't so infuriating it would be hysterical," she said.
If the Senate does subpoena Bolton, the White House is likely to cite executive privilege to try to send the matter to the courts and prevent him from testifying.
House investigators asked Bolton to testify but declined to force him to do so with a subpoena out of fear it would be tied up in the courts.
Bolton, who was fired by Trump in September after 17 months in the job, has since said he was prepared to testify before the Senate.