WASHINGTON — Sen. Lisa Murkowski announced Friday she will vote against hearing from witnesses in President Donald Trump's impeachment trial, likely dooming Democrats' hopes of hearing testimony from witnesses with firsthand knowledge of the president's conduct.

Murkowski's decision increases the likelihood that Trump's Senate impeachment trial will be the first in American history with no witness testimony.

"The House chose to send articles of impeachment that are rushed and flawed," Murkowski said in a statement. "I carefully considered the need for additional witnesses and documents, to cure the shortcomings of its process, but ultimately decided that I will vote against considering motions to subpoena."

She added, "Given the partisan nature of this impeachment from the very beginning and throughout, I have come to the conclusion that there will be no fair trial in the Senate. I don't believe the continuation of this process will change anything. It is sad for me to admit that, as an institution, the Congress has failed."

At this point, only two Republican senators are expected to vote in favor of hearing from witnesses – Mitt Romney of Utah and Susan Collins of Maine. Democrats had hoped Murkowski, who has bucked Trump in the past, join Romney and Collins. Republicans have a 53 seat majority in the 100-member Senate, and a majority vote is needed to approve witnesses.

Murkowski's decision means Democrats appear to have just 49 votes, barring any surprise switches from GOP senators.

If Murkowski had voted with the Democrats, the split would have been 50-50, and would have at least opened the door to the possibility, however unlikely, that Chief Justice John Roberts, who is presiding over the trial, could cast a tie-breaking vote.

Murkowski's statement said she didn't want to put Roberts in the position of deciding.

"It has also become clear some of my colleagues intend to further politicize this process, and drag the Supreme Court into the fray, while attacking the Chief Justice," she said. "I will not stand for nor support that effort. We have already degraded this institution for partisan political benefit, and I will not enable those who wish to pull down another."

Democrats needed to win over four Republicans in order to reach a 51 vote majority. But their hopes dampened early Friday morning when retiring Sen. Lamar Alexander, R-Tenn., announced he would vote against additional witnesses.

Another Republican Democrats hoped might vote their way, Sen. Rob Portman of Ohio, also announced on Friday that he was a no vote, even though he said he believes some of Trump's actions were "wrong and inappropriate."

"Our country is already too deeply divided and we should be working to heal wounds, not create new ones," Portman said.

Senators on Friday were spending several hours debating the issue of witnesses before the are expected to vote on the measure. The Senate then will vote on whether to convict or acquit Trump on the two articles of impeachment, although it was not clear when that vote will take place. Republican sources told NBC News the proceedings could last into next week.

Conviction requires a two-thirds vote of 67 senators. Anything less and the president is acquitted.

Democrats have argued that the Senate should hear from additional witnesses who have firsthand knowledge of Trump's conduct toward Ukraine, including Trump's former national security adviser John Bolton and acting chief of staff Mick Mulvaney.

Democrats stepped up their calls for testimony Sunday night after The New York Times reported that in an unpublished book, Bolton alleges that Trump told him nearly $400 million in aid to Ukraine would not be released until officials there offered assistance with investigations into Democratic targets, including former Vice President Joe Biden and his son, Hunter. Trump has denied Bolton's allegations.

NBC News has not seen a copy of the manuscript or verified the report, which cited multiple sources familiar with Bolton's account.