(CNN) In a sign of their importance, Senate Republicans gave the opening question on Wednesday to Sens. Susan Collins of Maine, Lisa Murkowski of Alaska and Mitt Romney of Utah, three Republicans who hinted they could vote to hear from witnesses. But everybody's talking about the lawmaker who stayed quiet, Lamar Alexander of Tennessee.

The senator right now holds perhaps more power than any other over the President, as the impeachment trial shifts from questions to voting as soon as Friday . Collins, Murkowski and Romney need at least one more Republican to join them if they want access to new information. Otherwise, the Senate-controlled GOP will block new witnesses from testifying and move to acquit the President.

Sen. Lamar Alexander, a Tennessee Republican who's retiring at the end of his term

It's unclear how Alexander will vote. On Thursday morning, he said he still hadn't made up his mind.

Collins, Murkowski and Romney asked multiple questions throughout the day trying to establish whether President Donald Trump held up aid to Ukraine because he wanted to damage his 2020 political rival, former Vice President Joe Biden. Romney wanted to know when Trump ordered to freeze the security assistance. Collins and Murkowski wondered whether Trump discussed Biden and Ukrainian corruption with his aides before the former vice president announced his 2020 campaign. Alexander, meanwhile, was mum. He took notes.

Before the questioning began, Tom Ingram, a prominent Tennessee Republican strategist who's known Alexander for more than 50 years, told CNN that his old boss views his role in the trial as a juror — "and jurors don't talk much." But over the past week, Alexander has appeared keenly interested in various arguments from the defense team.

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