Sen. Susan Collins, R-Maine, a key vote in the impeachment trial of President Donald Trump, will vote in favor of allowing witnesses to be called.
Collins, New England's sole Republican member of Congress, had previously taken the stance that she was "likely" to vote in favor of hearing new witnesses like former National Security Advisor John Bolton.
"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," Collins said in a statement. "Therefore, I will vote in support of the motion to allow witnesses and documents to be subpoenaed."
Fellow Republican Sens. Mitt Romney of Utah and Lisa Murkowski of Alaska have also expressed, to varying degrees, a desire to hear new witnesses testify.
If every Democrat votes in favor of hearing witnesses, they need four Republicans to join them. Retiring Sen. Lamar Alexander, R-Tenn., another key vote, announced Thursday night that he would vote against the measure.
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"Certainly, John Bolton is a key figure, and it may well be that he is who I decide needs to be called," Collins said earlier this week.
Chief Justice John Roberts could theoretically break a tie if Romney and Murkowski join Collins in breaking from the party rank.
A moderate Republican finishing her fourth term in the Senate, Collins is seen as particularly vulnerable to losing in the 2020 election. Her state is balanced between the two parties, and Democrats have been pushing to oust her since she voted to confirm Supreme Court Justice Brett Kavanaugh.