Ms. Sinema’s intentions are even more difficult to determine. She issued a statement at the outset of the trial saying she would “treat this process with the gravity and impartiality that our oaths demand” and has refused to talk to reporters ever since. She has been virtually silent on social media about impeachment.

But after Mr. Trump’s defense team wrapped up its opening statements on Tuesday, Ms. Sinema remained in the Senate chamber for more than 10 minutes, deep in conversation with Senator Lamar Alexander, Republican of Tennessee and a close ally of Mr. McConnell. On Wednesday, during the question-and-answer portion of the trial, she asked a question that seemed intended to undercut the president’s defense: Why was the aid to Ukraine withheld in secret?

“Kyrsten is in an interesting spot because she ran in her campaign as a moderate but also really stayed away from the partisan politics — she always referred to what Arizonans want, making it very local,” said Mike Noble, a Republican strategist in Arizona. “So she’s kind of doing this balancing act.”