(CNN) Democrats demanding new witnesses in Donald Trump's impeachment trial are seizing on a report that the President ordered former national security adviser John Bolton to keep military aid to Ukraine frozen in a bid to coerce political favors.

The revelation, first reported by the New York Times , heaped new pressure on a handful of Republican senators who may be interested in hearing testimony from Bolton and other witnesses in a process that could significantly delay Trump's hopes for a swift acquittal as soon as this week. A source with direct knowledge of the manuscript told CNN the Times' account of Bolton's account of the Ukraine aid hold discussion with Trump is accurate.

The new complications emerged on Sunday even as Trump was clearly sensing an imminent victory and showed he was spoiling for retribution against Democrats who led his impeachment in the House of Representatives last year.

In a series of late night tweets, Trump denied claims he told Bolton aid to Ukraine was tied to an investigation of the Bidens. "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. In fact, he never complained about this at the time of his very public termination," Trump tweeted

Trump's lawyers will Monday resume laying out their defense to Democratic charges that Trump abused his power by using the nearly $400 million in aid and the promise of a White House visit to wring dirt on political rivals from Ukraine.

The New York Times reported that a draft manuscript for Bolton's yet-to-be published book reveals that he was told by Trump to maintain a hold on military aid until officials in Ukraine opened investigations into former Vice President Joe Biden. There is no evidence of wrongdoing by either Joe Biden or his son, Hunter Biden, and Joe Biden has repeatedly defended his son and himself.

Such a revelation would bolster Democratic claims that Trump abused a public trust and taxpayer cash to try to leverage a foreign government's help in the 2020 election against one of his possible top opponents.

It would also undermine a key rationale of his defense team's argument that there were other reasons Trump withheld aid -- including his concern over corruption -- a claim challenged by substantial previous evidence.

House impeachment managers said in a statement that the Times report made it imperative that the Senate hears from Bolton.

"During our impeachment inquiry, the President blocked our request for Mr. Bolton's testimony. Now we see why," the managers said in the statement.

"The President knows how devastating his testimony would be, and, according to the report, the White House has had a draft of his manuscript for review. President Trump's cover-up must come to an end."

Three GOP sources told CNN's Manu Raju that party leaders in the Senate had expected to defeat a bid by Democrats to secure subpoenas of new witnesses in a vote this week. That prospect is now less certain, the sources said.

"The witness vote was always going to be tough," said one source involved in the strategy. "The story makes that clear again."

In another twist, Bolton's lawyer Charles Cooper said that the draft manuscript had been submitted to the National Security Council for a regular review to ensure it did not disclose classified material. But he suggested in a statement that the information reported by the Times meant the process had been "corrupted" and the document had been improperly accessed.

Before the reports about Bolton broke, there had been no signs yet that the four Republican votes needed for a Senate majority to back subpoenas would materialize. That being the case, the trial could end later this week, ushering a post-impeachment period in which the drama's political aftershocks will be crucial to November's election.

Democrats condemn Trump over 'threat' to Schiff

Trump's fury over impeachment indicates that he is unlikely to be magnanimous when the trial ends. On Sunday he launched a fierce attack on California Rep. Adam Schiff who is leading the Democratic case against him.

"He has not paid the price, yet, for what he has done to our Country!" Trump wrote on Twitter on Sunday.

"This is a wrathful and vindictive president. I don't think there's any doubt about it. And if you think there is, look at the President's tweets about me today, saying that I should pay a price," Schiff said.

But speaking on CNN's "State of the Union," Republican Sen. James Lankford, defended Trump.

"I don't think the President's trying to be able to do a death threat here or do some sort of intimidation," Lankford told CNN's Jake Tapper.

"It's no different than what Adam Schiff and what Speaker Pelosi were saying ... that folks will be paying a price at the ballot box, or that they will pay a price for this in the future, or that people hold them accountable for that."

Also on "State of the Union," California Rep. Zoe Lofgren said it was "unfortunate" that Trump had a tendency to say threatening things about people, adding he needed to "get a grip and be a little more presidential."

The debate about witnesses will define how the impeachment trial ends.

Trump's lawyers have two more days to lay out their case -- after which there will be a 16-hour period in which senators can submit questions to both legal teams. Only then will the question of whether to call witnesses potentially come to a vote.

Republicans are arguing that Democratic House leaders could have challenged Trump's blocking tactics to the courts and that it is not the Senate's job to continue investigating the case.

But Democrats counter that since Trump was threatening the fairness of the 2020 election they needed to act fast and censured the President's conduct in a second article of impeachment alleging obstruction of Congress.

They are also using the issue to paint the GOP as shielding a corrupt President and denying the American people a fair trial.

"If they're successful in depriving the country of a fair trial, there is no exoneration," Schiff said on "Meet the Press."

"There is no exoneration. Americans will recognize that the country did not get what the Founders intended because they put the word 'try' in the Constitution for a reason."

Democrats pile pressure on handful of Republicans over witnesses

Key senators to watch on the witness question as well as Romney include Maine Sen. Susan Collins, who faces a tough reelection race and has said she would "likely" vote to hear from new witnesses.

Alaska Sen. Lisa Murkowski, Tennessee's retiring Sen. Lamar Alexander and Colorado's Cory Gardner have also been in Democratic sights.

But before the New York Times report broke, Senate Democratic leader Chuck Schumer admitted on Sunday that he did not know whether Republicans would bow to the pressure.

"There's no good answer why there shouldn't be witnesses and documents, and no one has given one -- not the President's lawyers, not the Republican senators, not Mitch McConnell," Schumer said in a news conference in New York.

"But there's a lot of pressure not to do it, speed up the trial, do this, do that. But I am always hopeful," Schumer said.

Later on Sunday, Schumer said it was up to four Republican senators to make it happen.

"It's up to four Senate Republicans to ensure that John Bolton, Mick Mulvaney, and the others with direct knowledge of President Trump's actions testify in the Senate trial, Schumer tweeted.

House Speaker Nancy Pelosi called Trump's Ukraine conduct a "cover-up," tweeting: "The refusal of the Senate to call for him, other relevant witnesses, and documents is now even more indefensible."

Trump's legal team will resume their defense of the President when the trial reopens later on Monday.

"We don't believe that they have come anywhere close to meeting their burden for what they're asking you to do," White House counsel Pat Cipollone said. "In fact, we believe that when you hear the facts, and that's what we intend to cover today -- the facts -- you will find that the President did absolutely nothing wrong."

And they also turned the tables on Democratic claims that the President's actions were intended to interfere in the next presidential election, accusing Democrats of trying to overturn the 2016 election and to stack the decks against the President's reelection bid.