WASHINGTON (Reuters) - Democrats appeared to have fallen short on Thursday in their bid to garner the votes needed to call witnesses in President Donald Trump’s impeachment trial, clearing the way for his likely acquittal as early as this weekend.

Democrats have worked to get at least four Republicans to support the effort, but their hopes appeared dashed when Republican Senator Lamar Alexander, who had been undecided, declared further evidence in the case was unnecessary.

“There is no need for more evidence to prove something that has already been proven and that does not meet the United States constitution’s high bar for an impeachable offense,” Alexander said in a statement after the evening session.

Barring an unforeseen change of heart by another Republican senator, it appeared Alexander’s decision would bring a swifter conclusion to Trump’s two-week trial.

Republican Senator Susan Collins broke with her party and announced her support for witnesses. But Democrats needed at least of three of her colleagues to defect and give them the 51 votes needed to call witnesses and prolong the trial.

Late on Thursday, Republican Senator Lisa Murkowski said, “I am going to go reflect on what I have heard, reread my notes and decide whether I need to hear more.”

A 50-50 tie on the question of witness testimony and additional evidence could result if Murkowski and Senator Mitt Romney join Collins in backing the additional evidence.

Such a deadlock would mean that the drive for witnesses would fail, unless U.S. Supreme Court Chief Justice John Roberts, who is presiding over the Senate trial, voted to break the tie.

The backing of two-thirds of the Senate is required to remove Trump. He is unlikely to be convicted.

Democrats had hoped to hear from former National Security Adviser John Bolton after a report - which he has not denied - that he planned to say in an upcoming book that Trump told him he wanted to freeze $391 million in U.S. military aid for Ukraine until it investigated Democratic presidential contender Joe Biden and his son Hunter, who worked for a Ukrainian energy firm while his father was vice president.

The Democratic-controlled House of Representatives impeached Trump in December, formally accusing him of abusing his power for pressuring Ukraine to investigate the Bidens. The House also charged Trump with obstruction of Congress.

Trump’s acquittal would allow him to claim vindication just as the Democratic Party holds its first nominating contest for the Nov. 3 election in Iowa on Monday.

Trump held a rally in Des Moines, Iowa, on Thursday and slammed the trial, calling it an effort by Democrats to overthrow his 2016 election victory.

“They want to nullify your ballots, poison our democracy and overthrow the entire system of government,” Trump said.

TRYING TO UNMASK WHISTLEBLOWER

The two sides also sparred over the unnamed government official whose whistleblower complaint about Trump’s dealings with Ukraine spurred the drive for his removal.

Trump and some other Republicans have pressed for months to unmask the intelligence official who filed the report and have tried to paint the individual as a partisan figure working with Democrats to destroy Trump’s presidency.

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The government has provided security to the whistleblower in response to security threats, the individual’s lawyers have said.

On Thursday, the issue boiled to the surface again when Roberts refused to read a question from Republican Senator Rand Paul that included the name of a person right-wing media have accused of being the whistleblower.

Paul is one of several Republicans, including Trump, who have posted social media links to some of those news articles.

“The presiding officer declines to read the question as submitted,” Roberts said. He had rejected a similar question the day before.

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Paul said his query whether the individual named had worked with a member of Democratic Representative Adam Schiff’s staff to impeach Trump was not meant to unmask the whistleblower.

“My question’s not about the whistleblower,” he told reporters. “My question’s about two people who are friends.”

Democrats disagreed.

“This question was really framed and intended to expose the identity of the whistleblower and subject that whistleblower to retaliation,” Democratic Senator Richard Blumenthal told reporters.

Bradley P. Moss, a lawyer whose firm represents the whistleblower, called the Republican effort “a stain on the legacy of this constitutional republic.”

On Friday, each side is expected to present closing arguments before the Senate moves to the question of whether to call witnesses.

Schiff, the trial’s lead Democratic prosecutor in the trial, proposed both sides conduct closed-door witness depositions for a week while the Senate returns to normal business.

But there was no sign his plea was being considered by Republicans.

While Friday’s session could run late into the night, one senior Senate Republican aide said the vote on Trump’s acquittal could be engineered to take place during daytime hours on Saturday or even Monday.

That is when more Americans would be watching the outcome.