WASHINGTON — The impeachment trial of President Donald Trump moves to its final phase today.

The U.S. Senate voted 49 to 51 against allowing witnesses to testify in the trial, including former National Security Advisor John Bolton.

Republican senators Mitt Romney and Susan Collins joined Democrats in voting for witnesses, as they said they would. 

The Democrat-led effort to get more information from witnesses seemed to have collapsed late Thursday night, with a key Republican senator saying he won’t defect from his party and join Democrats in a crucial vote.

Retiring GOP Sen. Lamar Alexander was the Democrats' last hope to get former National Security Advisor John Bolton to testify under oath in Trump's impeachment trial.

They wanted his testimony because in a forthcoming book, Bolton says Trump told him he wanted to withhold military aid from Ukraine until it helped with investigations into Democratic rival and former Vice President Joe Biden. That assertion, if true, would undercut a key defense argument and go to the heart of one of the two articles of impeachment against the president.

But the Tennessee senator said late Thursday there's no need for Bolton, or anyone else, to testify, because the House impeachment managers have made their case.

In a statement, Alexander essentially argued that even if what Trump did was inappropriate, it didn’t rise to the level of removing the 45th president from office.

Now Senate Majority Leader Republican Mitch McConnell can push ahead for a vote to either convict or acquit. With Republicans sticking together, acquittal is all but guaranteed at this point. 

"But I would note that even in Sen. Alexander's statement announcing his opposition to new evidence, he said that it was proven the president did what he was accused of," Senate Minority Leader Sen. Chuck Schumer said.

Senate Democrats vowed not to go down without a fight. They're expected to introduce a series of motions that will extend the trial and put vulnerable Republicans up for reelection on the record on the issue of witnesses.

Final arguments are scheduled for Monday. The final voting would be Wednesday, the day after Trump's State of the Union speech.

Live updates from our reporters in DC