This was signaled by the publication Sunday night on the New York Times website of an article based on the unpublished manuscript of a forthcoming book by Trump's former national security adviser, John Bolton.
The House impeached Trump on charges of abusing the powers of his office by asking Ukraine to investigate Biden and of obstructing a congressional inquiry into his conduct.
Senate Democrats tried last week to subpoena Bolton to testify, but Republicans killed all such efforts. The makeup of the chamber has focused attention on just a few Republicans who are considered open to breaking from the party ranks and voting with Democrats.
"They're asking you not only to overturn the results of the last election, but they're asking you to remove President Trump from the ballot in an election that's occurring in approximately nine months", Pat Cipollone told senators.
Bolton's lawyer released a statement Sunday evening in which said that the NSC's "prepublication review process has been corrupted". Trump's legal team has repeatedly insisted otherwise, and Trump tweeted on Monday that he never told Bolton such a thing.
Significantly, Schiff raised the threat only at the end of his interview, spending most of his time on the program defending USA intelligence agencies against criticisms made by Trump's defense counsel during the two-hour Senate session on Saturday morning, where the defense began its rebuttal of the House impeachment case. Still, Bolton has said he would appear if issued a subpoena by the Senate.
Republicans "are refusing to hold a fair trial, refusing the Senate access to White House documents and refusing to hear from valuable witnesses", said Rahna Epting, executive director or MoveOn.
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With Republicans holding a 53-47 majority in the Senate, it is unlikely that the president will be removed from office.
Bolton's lawyer blamed the White House for the disclosure of the book's contents.
He confirmed the had sent a copy of the book to the White House on December 30 - 12 days after Mr. Trump was impeached by House Democrats - for review for classified information, adding though that Bolton believed it contained no sensitive in terms of national security data.
House impeachment manager Adam Schiff (D-Calif.) speaks during impeachment proceedings against President Donald Trump in the Senate at the U.S. Capitol in Washington on January 22, 2020.
Bolton left his post in September.
"The problem with John is, that it's a national security problem", he told reporters at an impromptu press conference at the World Economic Forum in Davos, adding that Bolton "knows some of my thoughts, what I think about leaders - what happens if he reveals what I think about a leader and it's not very positive?" The Senate is now conducting a trial to determine whether to convict Trump under the articles of impeachment. But the newness of this information, and Bolton's proximity to Trump himself, makes it harder for Trump's lawyers to kick up enough dust for Republican senators to pretend there's still room for reasonable doubt about Trump's motives for withholding aid to Ukraine.
The seven House impeachment managers, led by Intelligence Committee Chairman Adam B. Schiff, also pushed for Bolton to be called to testify.
Trump disputed the reports in a series of tweets early Sunday morning. Trump has denied making the statement to Bolton. Now it's in the Senate GOP's hands, and you can bet it's a tougher call now.
Sen. John Thune, a member of GOP leadership, said the Bolton news is a "surprising" development for many of his colleagues and that most are "keeping their powder dry" at this point.