And it is not clear how much damage three weeks in Washington would do to Democratic senators, who would still have long weekends to campaign. The dog days of August are typically not when voters are tuned in to midterm elections.

Lawmakers face a Sept. 30 deadline to pass spending legislation to avoid a government shutdown on Oct. 1, when the 2019 fiscal year begins. This year, it took lawmakers until March to pass a broad spending bill for the 2018 fiscal year, which was already almost halfway over. At the time, Mr. Trump expressed dissatisfaction and said he would never again sign a similar measure. For the 2019 fiscal year, lawmakers are trying to pass spending legislation in pieces, and in a more timely fashion.

On Tuesday, Democrats, trying to make the most of a scheduling decision that leaves them at a disadvantage, emerged from their weekly policy luncheon and tried to pressure their Republican colleagues to dedicate the reclaimed time in August to addressing rising health care and prescription drug costs.

Senator Chuck Schumer of New York, the Democratic leader, told reporters that working through August gave the Senate a “perfect opportunity” to expand Medicare access, increase tax credits for families purchasing health insurance and create a national reinsurance program aimed at lowering insurance premiums. He said he expected Mr. Trump to skip his August vacation, as well, and to remain in Washington.

Senators most affected by the schedule change simply shrugged — at least publicly.

“Frankly, the best thing I can do for the people of Wisconsin is fight to lower health care costs, fight to lower prescription drug costs — and we would look forward to the opportunity to deliver results on health care policy during the August recess,” said Senator Tammy Baldwin, Democrat of Wisconsin, when asked if the schedule change would hurt her campaign.

Senator Heidi Heitkamp of North Dakota, another vulnerable Democrat, said: “We were sent here to vote, we were sent here to do our job. I’m perfectly fine with it.”

Senator Joe Manchin III of West Virginia, a moderate Democrat who is running for re-election in a state in which Mr. Trump won roughly two-thirds of the vote, said he would like to see Senate leaders go further.

“We ought to start working on Mondays and Fridays, too,” he said. “I always thought you had a workweek Monday to Friday. I never did get a four-week vacation, so I never did understand that here.”