WASHINGTON — House Democrats were prepared and passionate as they laid out over three draining days in the Senate a detailed argument for impeaching and removing President Trump. What they do not appear to have been is persuasive to the critical handful of Republicans who hold the key to their immediate objective: gaining access to witnesses and documents that could cement their case.

While Senate Republicans commended Democrats for a thorough and mainly respectful presentation, they gave no indication as the arguments drew to a close that they were ready to expand the scope of the trial, let alone to vote to remove Mr. Trump from office at the end of it. The sense in the Capitol was that the trial was heading toward its predictable conclusion, Mr. Trump’s acquittal, as early as next week.

It has always been only a small group of Republicans who were truly considered possible defectors who might join Democrats on the volatile issue of seeking witnesses in the trial. Senators Susan Collins of Maine and Mitt Romney of Utah have been the most forward-leaning of the Republicans who have expressed openness to the idea. But their ranks appeared to be thinning this week.

Senator Lamar Alexander, Republican of Tennessee, who had signaled potential interest in considering new evidence, said on Friday he would wait until after the president’s defense and senators have time to question the lawyers to make his decision, but he did not sound eager to push the proceeding into the unknown.