Republican leaders do not yet have the votes to block Democrats' demand for more witnesses at President Donald Trump's impeachment trial, Senate Majority Leader Mitch McConnell conceded to fellow GOP senators late Tuesday. It could be a major hurdle for Trump's hopes to end the trial with a quick acquittal.

Democrats are demanding several witnesses, especially John Bolton, Trump's former national security adviser who writes in a forthcoming book that Trump told him he wanted to withhold military aid from Ukraine until it helped with investigations into Democratic rival Joe Biden. That's the crux of one major article of impeachment against the president.

McConnell gave the news to senators, according to a Republican familiar with a closed-door meeting of GOP senators and granted anonymity to discuss it.

McConnell convened the meeting shortly after Trump's legal team made its closing arguments in the trial.

There are still several days before any potential witness vote would be taken. A decision to call more witnesses would require 51 votes to pass. With a 53-47 majority, Republicans can only afford to lose three.

If senators agree they want more witnesses they would then have to vote again on which ones to call.

The news came as Trump's legal team argued forcefully against the relevance of testimony from Bolton and concluded their defense as the Senate braced for debate on witnesses.

Who's Who in the Trump-Ukraine Affair

President Donald Trump faces a formal impeachment inquiry led in the Democratic-controlled House after he asked the newly elected Ukrainian president to investigate one of his chief political rivals, former Vice President Joe Biden.

While scoffing at Bolton's manuscript, Trump and the Republicans have strongly resisted summoning Bolton to testify in person about what he saw and heard as Trump's top national security adviser.

Senate Republicans spent two days behind closed doors discussing ideas to satisfy those who want to hear more testimony without prolonging the proceedings -or jeopardizing the president’s expected acquittal.

Those lost steam, and Democrats showed no interest.

Chuck Schumer, the Senate's top Democrat, called a proposal for senators to be shown the manuscript in private, keeping Bolton out of public testimony, "absurd."

"We're not bargaining with them. We want four witnesses, and four sets of documents, then the truth will come out," Schumer said.

Senators are being warned that if they agree to call Bolton to testify or try to access his book manuscript, the White House will block him, beginning a weeks-long court battle over executive privilege and national security. That had seemed to leave the few senators, including Sen. Mitt Romney of Utah, Susan Collins of Maine and Lisa Murkowski of Alaska, who have expressed a desire to hear new testimony without strong backing.

Also, other Republicans including Sen. Pat Toomey want reciprocity -- bring in Bolton or another Democratic witness in exchange for one from the GOP side. Some Republicans want to hear from Biden and his son, who was on the board of a Ukrainian gas company when his father was vice president.

A day after the defense team largely brushed past Bolton, attorney Jay Sekulow addressed the controversy head-on by dismissing his manuscript — said to contradict a key defense argument about Trump's dealings with Ukraine — as "inadmissible."

More Republicans are calling for witness testimony in the impeachment trial of President Trump following new accusations from former National Security Advisor John Bolton.

"It is not a game of leaks and unsourced manuscripts," Sekulow said.

The argument built on a separate one Monday night from Trump attorney Alan Dershowitz, who said that nothing in the manuscript — even if true — rises to the level of an impeachable offense. Sekulow also sought to undermine the credibility of Bolton's book by noting that Attorney General William Barr has disputed comments attributed to him by Bolton.

The legal team also delved into areas that Democrats see as outside the scope of impeachment, chastising former FBI Director James Comey and seizing on surveillance errors the FBI has acknowledged making in its Russian election interference probe.

Trump's attorneys argued that the Founding Fathers took care to make sure that impeachment was narrowly defined, with offenses clearly enumerated.

"The bar for impeachment cannot be set this low," Sekulow said. "Danger. Danger. Danger. These articles must be rejected. The Constitution requires it. Justice demands it."

Before consideration of witnesses, the case now moves toward written questions, with senators on both sides getting 16 hours to pose queries. By late in the week, they are expected to hold a vote on whether or not to hear from any witnesses.

"I don't know that the manuscript would make any difference in the outcome of the trial," said Roy Blunt of Missouri, a member of GOP leadership. And some Republicans said they simply don’t trust Bolton’s word. Rand Paul of Kentucky called Bolton "disgruntled" and seeking to make money off his time at the White house.

John Kelly, Trump's former White House chief of staff, told an audience in Sarasota, Florida, that he believes Bolton.

White House officials privately acknowledge that they are essentially powerless to block the book's publication, but could sue after the fact if they believe it violated the confidentiality agreement Bolton signed against disclosing classified information.

Democratic House managers addressed the media on Tuesday after President Donald Trump’s defense team wrapped up their arguments in the Senate impeachment trial.

Trump is charged with abusing his presidential power by asking Ukraine's leader to help investigate Biden at the same time his administration was withholding hundreds of millions of dollars in security aid. A second charge accuses Trump of obstructing Congress in its probe.

Trump and his lawyers have argued repeatedly that Democrats are using impeachment to try to undo the results of the last presidential election and drive Trump from office.

On Tuesday, as he was resting his case, Cipollone played video clips from House Democrats during the presidential impeachment of Bill Clinton — including several who are now managers of the Trump impeachment trial — in an attempt to depict them as hypocritical for sounding the alarm then about the partisan dangers of impeachment.

"What they are asking you do is to throw out a successful president on the eve of an election, with no basis, and in violation of the Constitution," Cipollone said. "Why not trust the American people with this decision? Why tear up their ballots?"

Democrats, meanwhile, say Trump's refusal to allow administration officials to testify only reinforces that the White House is hiding evidence. The White House has had Bolton's manuscript for about a month, according to a letter from Bolton's attorney.

No matter the vote on witness, acquittal still seems likely given that Republicans hold a 53-47 majority in the Senate and conviction would require a two-thirds majority.

According to data compiled by C-SPAN, the House managers used just under 22 of their 24 hours over three days, while the White House team used almost 12 hours, or half their time.