"I think it's very likely I'll be in favor of witnesses, but I haven't made a decision finally yet and I won't until the testimony is completed," the Utah Republican said, following the first day of the Trump team's opening arguments.
Romney, a conservative who has before expressed frustration with Trump, previously indicated that he would be interested in hearing testimony from former national security adviser John Bolton. He said earlier this month, "I would like to hear from John Bolton and other witnesses, but at the same time I'm comfortable with the Clinton impeachment model when we have opening arguments first and then we have a vote on whether to have witnesses."
Democrats need four Republicans to vote with them in favor of subpoenas for witnesses or new evidence in order to extend the trial and gather new information.
There are only maybe four, realistically, who would vote in favor of calling witnesses who could testify against the President. That short list includes Romney, relative moderates Sens. Susan Collins of Maine and Lisa Murkowski of Alaska, and endangered senators up for reelection like Sen. Cory Gardner of Colorado.
Collins and Murkowski have also signaled that they're open to hearing from witnesses, should they feel it's needed after opening arguments.
Senate Majority Leader Mitch McConnell blocked Democratic efforts to push for an initial deal to subpoena witnesses and documents until after opening statements and questioning.
The Republican leader now argues that new witness testimony would raise constitutional concerns -- a sign GOP sources told CNN that McConnell plans to bring a swift end to the proceeding.