Senate Majority Leader Mitch McConnell told fellow Republicans that he did not have the votes to stop Democrats from calling witnesses in the Trump impeachment trial.
McConnell and the president's defense team oppose calling witnesses, including former national security adviser John Bolton who claims in an upcoming book that Trump directly told him that he was withholding military aid to Ukraine in exchange for a political favor.
Fifty-one of the 100 senators hearing the impeachment case would have to vote in favor of witnesses — meaning just four Republicans would have to side with the 47 Democrats and independents.
Several moderate Republicans, including Mitt Romney, Susan Collins and Lisa Murkowski, have said they may be interested in hearing Bolton and others testify.
Bolton's revelation in his soon-to-be-published book could blow up the White House's major defense that the president did nothing wrong in withholding $391 million in military aid to Ukraine.
Trump's lawyers say he had the right to freeze the aid because he was concerned about Ukrainian corruption and wanted Europe to pitch in more to help Ukraine fight Russian-backed separatists.
Democrat impeachment managers say Trump abused his power and would not release the aid until Ukrainian President Volodymyr Zelenskiy publicly committed to investigating Trump's rival for the 2020 White House, Democrat Joe Biden, for alleged corruption.
Defense: 'No quid pro quo'
The Trump defense team wrapped up its case Tuesday. Lawyer Jay Sekulow told the 100 senators acting as jurors, "We are clear in our position there was no quid pro quo" in Trump asking Zelenskiy to investigate Biden.
Sekulow said Trump had "a proper government interest" to investigate possible corruption in asking for the Ukraine investigations of Biden, his son Hunter's work at a Ukrainian natural gas company, and a debunked theory that Ukraine meddled in the 2016 U.S. presidential election that Trump won. No evidence has emerged of wrongdoing by the Bidens.
"He knew what he was saying," Sekulow said of Trump's July 25 conversation with Zelenskiy. "To say he's not acting in the national interest is wrong."
Sekulow's arguments came on the third and last day of the president's defense that he should be acquitted of two articles of impeachment — abusing the power of the presidency and obstructing congressional efforts to investigate his Ukraine actions.
"These articles must be rejected," Sekulow said. "The Constitution requires it. Justice demands it."
White House counsel Pat Cipollone concluded, "This should end now, as quickly as possible."
"Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense," famed attorney Alan Dershowitz said late Monday.
A four-hour debate is expected Friday on whether to subpoena Bolton, whom Trump dismissed in September, and other witnesses familiar with Trump's Ukraine-related actions and documents held by the White House and government agencies.
Trump's Republican defenders have balked at calling Bolton as a witness.
"I think, at this point, if you want to hear from John Bolton, you have to ask yourself, 'Is he a disinterested party? Is he a neutral party? Or is he someone who's very unhappy — disgruntled, fired employee who now has a motive, a multimillion-dollar motive, to inflame the situation?'" asked Senator Rand Paul.
With the completion of Trump's defense, senators will have 16 hours over the next two days to question Trump's legal team and House managers who have prosecuted the case against the president.
Trump has often claimed that his call last year to Zelenskiy was "perfect."
Trump's lawyers sharply attacked Biden and his son's lucrative work for a Ukrainian natural gas company. They accused Democrats of improperly using impeachment as a weapon to get rid of a president they simply don't like, to overturn the 2016 election, and keep Trump off the ballot in November as he seeks a second term in the White House.
A two-thirds vote in the 100-member Senate is needed to convict Trump and remove him from office. But with Republicans holding a 53-47 majority and no Republican calling for his ouster, Trump is all but assured of being acquitted.