U.S. senators voted against calling witnesses in President Donald Trump's Senate impeachment trial on Friday, paving the way for an expected acquittal by the Republican-majority Senate.

The Democrats' drive to hear testimony from witnesses, like former national security adviser John Bolton, was defeated by a vote of 51-49. Bolton is thought to have first-hand knowledge of the president's efforts to pressure Ukraine to investigate a political rival, former vice-president Joe Biden.

Republican Senators Mitt Romney and Susan Collins voting in favour of hearing from witnesses. Earlier, Alaska Sen. Lisa Murkowski said she carefully considered the need for witnesses and documents to be used in the trial that will determine whether Trump is removed from office, but ultimately decided against it.

The Senate will hear closing arguments in the trial on Monday, and a final vote will be held at 4 p.m. ET on Wednesday, according to a resolution from Senate Majority Leader Mitch McConnell that passed 53 to 47 on Friday night. Democrats proposed four amendments to McConnell's resolution, but all four failed.

Chief Justice of the United States John Roberts reads the result of a vote that saw Democrats' call for witnesses in the Senate impeachment trial fail. (Senate Television/The Associated Press)

The Senate is almost certain to acquit Trump of the impeachment charges, as a two-thirds Senate majority is required to remove Trump and none of the chamber's 53 Republicans have indicated they will vote to convict.

Senate Minority Leader Chuck Schumer called the result of the witness vote a "perfidy," and "a tragedy on a very large scale."

Schumer, speaking outside the chamber shortly after the vote, said the Senate went along with a "sham trial," adding that an acquittal will have "no value."

'The Senate turned away from truth and went along with a sham trial,' Senate Minority Leader Chuck Schumer told reporters. (Mark Wilson/Getty Images)

The Senate resumed debate on Friday afternoon, with further arguments from Trump's legal team and the House of Representatives Democrats serving as prosecutors.

Republican Sen. Lamar Alexander of Tennessee, who had been undecided, said late on Thursday that Democrats had proven the case against Trump but that the president's actions did "not meet the United States Constitution's high bar for an impeachable offence."

Alexander said it was "inappropriate for the president to ask a foreign leader to investigate his political opponent and to withhold United States aid to encourage that investigation."

Murkowski said she was swayed to reject witnesses in part by the "deficiencies" in the House impeachment process that brought the two impeachment articles to the Senate.

"We started with a flawed product, and I'm at that point where I'm frustrated and disappointed, angry at all sides," she told reporters at the end of Friday's session.

Republican Sen. Marco Rubio said that for him, Trump's actions didn't rise to the level of impeachable offences.

He said voters will be able to review the trial record themselves and decide in the November election what they want to do about it.

"Every single statement that was made, every allegation ... are all available on the record for the American people to take into account in just nine months, and that is a far preferable remedy than to remove him in a process that half the country is going to determine is illegitimate and coup d'etat," he told reporters Friday night.

'Guard rails will be removed': Blumenthal

The Democratic-controlled House voted to impeach Trump on Dec. 18, formally charging him with abuse of power for asking Ukraine to investigate Biden, as well as a discredited theory about a computer server in connection with the 2016 election..

The House also charged Trump with obstruction of Congress for blocking current and former officials from providing testimony or documents. Trump is only the third president in U.S. history to be impeached.

Senate Democrats have been arguing throughout the two weeks of proceedings that lawmakers need to hear from witnesses in order for it to be a fair trial. This would be the first Senate impeachment trial in U.S. history with no witnesses, including trials of two prior presidents and a number of other federal officials.

Democratic Sen. Richard Blumenthal of Connecticut told CNN: "The danger going forward is that the guard rails will be removed. The president now feels, in effect, legally unleashed." Blumenthal added that the trial is no vindication for Trump "because it was really no trial — no witnesses, no documents, no real evidence."

Trump's Republican allies have sought to keep the trial on a fast track and minimize any damage to the president, who is seeking re-election on Nov. 3. Trump denies any wrongdoing.

Trump's acquittal would allow him to claim vindication just as Democrats hold the first of the state-by-state nominating contests on Monday in Iowa to choose the party's nominee to challenge Trump in the election. Biden is a leading contender to face Trump in the November vote.

The president held a rally in Des Moines, Iowa, on Thursday night and denounced the impeachment trial, again calling it an effort by Democrats to overturn his 2016 election victory.

He is scheduled to give the State of the Union address on Feb. 4.

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Contradicting Trump's version of events, Bolton wrote in an unpublished book manuscript that the president told him he wanted to freeze $391 million US in security aid to Ukraine until Kyiv pursued investigations of Democrats, including Biden and the former vice president's son, Hunter Biden, the New York Times reported.

Bolton's allegations go to the heart of impeachment charges against Trump. Democrats have said Trump abused his power by using the security aid — passed by Congress to help Ukraine battle Russia-backed separatists — as leverage to get a foreign power to smear a political rival.

U.S. Secretary of State Mike Pompeo, the highest-ranking U.S. official to travel to Ukraine since the impeachment began, emphasized American support for Ukraine.

"Today I'm here with a clear message: the United States sees that the Ukrainian struggle for freedom, democracy and prosperity is a valiant one. Our commitment to support it will not waver," Pompeo said in Kyiv.

Ukraine counts on the United States for diplomatic support, sanctions on Moscow and military aid to buy Javelin anti-tank missiles and other hardware as it battles Russian-backed separatists in a war that has killed more than 13,000 people.