The Senate on Friday voted against calling witnesses in President Donald Trump's historic impeachment trial.

The motion to call witnesses failed, with 51 senators voting against it and 49 voting in favor.

Democrats repeatedly made the case that former national security adviser John Bolton should be called to testify.

The Senate called witnesses in all 15 previous impeachment trials in US history, including those of former Presidents Andrew Johnson and Bill Clinton.

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The Senate on Friday voted against calling witnesses in President Donald Trump's impeachment trial.

The motion for witnesses failed, with 51 senators voting against the measure and 49 voting in favor of it.

Fifty-one senators needed to vote in favor of calling witnesses for the motion to pass. There are 45 Democrats, two independents who caucus with Democrats, and 53 Republicans in the Senate.

That means four Republican senators needed to side with the Democratic caucus for the Senate to call witnesses.

In the Republican caucus, only Sens. Mitt Romney of Utah and Susan Collins of Maine voted for additional witnesses.

Sen. Lamar Alexander of Tennessee, once considered a possible pro-witness Republican, announced on Thursday night that he would not vote in favor of witnesses.

Sen. Lisa Murkowski of Alaska, another closely watched potential swing vote on the matter, confirmed Friday afternoon that she would also vote against calling more witnesses.

The highly anticipated vote came after a fierce debate over whether to call witnesses on Friday.

The seven Democratic House impeachment managers, who act as prosecutors in the trial, said the Senate has a duty to call additional witnesses, such as former national security adviser John Bolton, who have firsthand evidence against Trump.

The House of Representatives impeached Trump last month on charges of abuse of power and obstruction of Congress. The two articles of impeachment relate to the president's efforts to strong-arm Ukraine to deliver politically motivated investigations targeting his rivals.

While doing so, Trump withheld $391 million in military aid to Ukraine and leveraged a White House meeting that Ukrainian President Volodymyr Zelensky desperately sought and still hasn't gotten.

Calls for Bolton to testify intensified after The New York Times reported that the former national security adviser wrote in a forthcoming book that the president personally told him he would not release Ukraine's military aid until the country publicly committed to pursuing the investigations Trump wanted.

Earlier on Friday, The Times said in another report that Bolton also said Trump asked him during a meeting in May to call Zelensky and ensure he met with Rudy Giuliani, Trump's personal lawyer, when Giuliani planned to travel to Ukraine that month.

Giuliani told The Times last year that he was going to go to Ukraine to push the government to pursue the investigations targeting former Vice President Joe Biden and the Democratic Party.

Around the time of the May meeting that Bolton outlines in his upcoming book, Giuliani also sent a letter to Zelensky asking to meet with him on May 13 or 14. He didn't indicate what he specifically wanted from the meeting, but Giuliani emphasized in the letter that he was requesting the meeting in his capacity as Trump's personal lawyer.

"Just to be precise, I represent him as a private citizen, not as President of the United States," Giuliani wrote. "This is quite common under American law because the duties and privileges of a President and a private citizen are not the same."

The Senate called witnesses in all 15 impeachment trials in US history, including those of former Presidents Andrew Johnson and Bill Clinton.

The catalyst for Trump's impeachment was a whistleblower complaint from an anonymous US intelligence official who accused the president of soliciting foreign election interference during a July 25 phone call with Zelensky.

During the call, Trump personally urged Zelensky to launch inquiries into the Bidens and a debunked conspiracy theory that Ukraine interfered in the 2016 election.

Biden is a leading contender for the 2020 Democratic nomination, and there's no evidence of wrongdoing or illegal activity on his or his son Hunter's part in Ukraine.

Trump eventually released Ukraine's military aid in September, less than a week after three House committees launched investigations into his dealings.