Washington (CNN) Sen. Mitt Romney said Saturday that it's "very likely" that he'll be in favor of calling witnesses in the impeachment trial of President Donald Trump, but won't decide until after opening arguments.

"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

Asked if he thought the defense team was effective, Romney replied, "I just don't have any comments on the process or the evidence until the trial is over."

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.

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