“One way or the other, the Mueller hearing will be a turning point with respect to the effort to hold Donald Trump accountable for his reckless, degenerate, aberrant and possibly criminal behavior,” said Representative Hakeem Jeffries of New York, the House Democratic Caucus chairman and a member of the Judiciary Committee. “After the hearing, we will be able to have a better understanding of the pathway forward concerning our oversight responsibilities and the constitutional tools that are available to us.”

Partisans in both parties may already have made up their minds, but Democrats are counting on Mr. Mueller’s testimony to focus the broader public’s attention on the findings of his 22-month investigation — either to jumpstart a stalled impeachment push or electrify the campaign to make Mr. Trump a one-term president.

Even Speaker Nancy Pelosi, who has been a voice of caution on impeachment for much of the year, has tied the testimony to Democrats’ broader political prospects.

“This coming election, it is really an election that the fate of this country is riding on,” she told House Democrats at a private meeting recently, according to an aide who was there. “This presidency is an existential threat to our democracy and our country as we know it.”