In Washington, a man waited for Ms. Collins in the dark as she parked her car one evening in the pouring rain, then followed her several blocks to her townhouse. A neighbor lamented to her that he hated living next to “a rape apologist.”

“It just made the whole time very unpleasant,” said Ms. Collins, who faces a steep re-election challenge in November, when she will seek a fifth term. “But anyone who thinks that they can intimidate me doesn’t know me.”

Ms. Collins’s history, and her competitive race to keep her seat, puts her at the top of the list of senators under scrutiny in the impeachment debate. Reporters on Capitol Hill toil to divine her intentions; as the articles of impeachment were read Thursday on the Senate floor, Ms. Collins, who was suffering a bad cold, coughed and dabbed at her eyes, causing several reporters to call her office to ask why she was crying.

Her vote at the conclusion of Mr. Trump’s trial almost certainly will not determine whether he becomes the first president to be removed from office by the Senate — the 67 votes required, at this point, are not there. But Ms. Collins will be pivotal to resolving procedural questions, including a battle between Republicans and Democrats over calling witnesses and admitting new documents as part of the Senate trial. And Ms. Collins has signaled that she will buck her party and support both moves.

“I would anticipate that it is likely that I would vote to have more information brought forward, whether witnesses or documents or both,” she said Thursday.

Ms. Collins convened several meetings in her office with the Republican senators Lisa Murkowski of Alaska, Mitt Romney of Utah and Lamar Alexander of Tennessee to cobble together a provision ensuring a vote on the matter after opening arguments by both sides and questions from senators. If the four hang together on the issue, their votes would be enough — along with the 47 that Democrats control — to demand more information come out in the trial.

And her eventual vote on whether to remove the president will be politically significant as well, both to Mr. Trump and to Ms. Collins. Since he was impeached last month, the president has leaned on his party’s unified opposition to dismiss the whole effort as a partisan “hoax,” and he has made it clear he looks forward to an exoneration by the Republican-led chamber. Even a single Republican vote in favor of removing him would undermine that.