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WASHINGTON — Two Republican senators who had publicly toyed with whether to vote to call witnesses in President Donald Trump's impeachment trial have split on the issue, endangering Democratic hopes to get former national security adviser John Bolton and others to testify before the Senate. Republican Sen. Susan Collins of Maine said Thursday night she will vote in favor of additional witnesses in Trump’s trial, but her announcement was undercut almost immediately by Republican Sen. Lamar Alexander, who said he will vote against it.

Both Collins and Alexander had been closely watched in the lead-up to the witness vote, which is expected to take place Friday. But Alexander's decision means it will be very tough for Democrats to get the 51 votes they need to bring in witnesses. In total, Democrats will need four Republicans to join them for the vote to succeed.

Republican Sens. Lisa Murkowski and Mitt Romney have both said they would also be open to additional witnesses. Murkowski said she'll make her decision public on Friday. But even if both senators agree, it's unclear where the other Republican vote would come from.

Collins argued in a statement Thursday night that the Senate should hear from additional witnesses and be able to subpoena documents in the case. "I believe hearing from certain witnesses would give each side the opportunity to more fully and fairly make their case, resolve any ambiguities, and provide additional clarity," she said. But Alexander, a Tennessee Republican who is retiring at the end of this year, tweeted, "there is no need for more evidence to prove something that has already been proven and that does not meet the U.S. Constitution’s high bar for an impeachable offense."

Drew Angerer / Getty Images Sen. Lamar Alexander