The U.S. Senate on Friday voted against calling new witnesses in the impeachment trial of President Donald Trump, setting the stage for Trump's likely acquittal next Wednesday on the back of his Republican Party's majority in the chamber.

Trump, 73, became the third president in U.S. history to be impeached when the House of Representatives in December moved to charge him over allegations that he pressured a foreign country to investigate a key political rival in his re-election bid.

It requires a two-thirds majority of the 100-member Senate to convict the president and remove him from office. But the Republicans, who hold 53 seats in the chamber, are united behind Trump, making his conviction unlikely.

The Republicans have been eager for a swift end to the trial, which started Jan. 16 with Trump denying any wrongdoing over the two charges he has faced, namely abuse of power and obstruction of Congress in its investigation.

Fresh revelations by new witnesses such as former National Security Adviser John Bolton, who has allegedly claimed to have direct evidence to support the allegations, could have been damaging to the president ahead of the November election.

Trump has been under fire over a July phone call with Ukrainian President Volodymyr Zelenskyy in which, according to a transcript released by the White House, Trump asked Zelenskyy to "do us a favor" by looking into some "horrible" information he heard about former Vice President Joe Biden and his son, who had served on the board of a purportedly corrupt Ukrainian energy company.

A group of House Democratic lawmakers serving as prosecutors in the trial have asserted that Trump abused the power of his office by pressuring Ukraine to announce investigations into the Bidens by withholding congressionally approved military aid to the country, which faces aggression from Russia.

Biden, who served under former Democratic President Barack Obama, has been one of the frontrunners in the race for his party's presidential nomination.

Trump's defense team has fully contested the allegations, saying that the prosecutors have no direct evidence that Trump linked the military aid to any investigations, and that their case rests almost entirely on the "presumptions" of a key U.S. diplomat and speculation from other witnesses.

The Democrats stepped up their calls to hear from witnesses, especially after The New York Times reported Sunday that the manuscript of Bolton's upcoming book showed Trump directly told his then national security adviser in August that he wanted to continue freezing the aid until Ukraine helped with investigations, including of the Bidens.

Trump, who fired Bolton in September over disagreements on foreign policy issues, has tweeted that he never told Bolton such a thing and speculated that Bolton was only trying to "sell" his book.

House Democrat lawmaker Adam Schiff, who has led the investigation, said Friday, "The facts...will continue to come out. And the question before you today is whether they will come out in time for you to make a complete and informed judgment as to the guilt or innocence of the president."

"A trial is supposed to be a quest for the truth. Let's not fear what we will learn," he added.

But Trump's defense team shrugged off the need to do so, saying that testimonies of witnesses presented by the Democrats, which they themselves have said already prove their case, are enough for the senators to reach a decision.

A motion to subpoena witnesses and documents was rejected by a vote of 51-49, with Democrats falling short of convincing four Republicans they needed to form a majority on the issue.

As for the charge of obstruction of Congress, the Democrats have said Trump directed his administration officials to defy every subpoena issued in the impeachment investigation by the House.

Trump's lawyers insisted that he acted properly upon advice from the Justice Department and blamed the refusal to comply with some subpoenas on what they see as a lack of authorization from the House that initially launched an investigation without a vote.

The Senate decided Friday to hold a vote on the two articles of impeachment at 4 p.m. Wednesday.

In U.S. history, only two other presidents have been impeached -- Andrew Johnson in 1868 and Bill Clinton in 1998. Clinton was charged with obstructing justice and lying under oath about his extramarital affair with a White House intern.

No president has been removed from office as a direct result of the impeachment process.

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