When President Trump announced that he would hold an unusual party convention just weeks ahead of the midterm elections, he wrote online that it would be “a truly Historic Event” and “a RALLY like none other!”

But as they stare down a bleak political environment, with the president’s approval rating at a record low, many House and Senate Republicans running in competitive races across the country are keeping their distance from Mr. Trump’s Dallas pep rally next week, choosing to stay home and campaign instead.

“I’ve actually never attended any convention,” said Representative David Valadao of California, who was already one of the most vulnerable Republicans in the House when his district was redrawn last year to include more Democratic voters from Fresno.

Mr. Valadao said he would not be breaking his streak for this gathering on Sept. 9 and 10, which lawmakers are being charged $25,000 to attend. Mr. Trump will headline the proceedings on both nights.

“I was invited, but staying home to campaign and spend time with family,” said Representative Max Miller of Ohio, the embattled Republican fighting to keep his seat after allegations of domestic abuse of his ex-wife and toddler daughter upended his campaign.

Representative María Elvira Salazar of Florida, the former Telemundo host who represents an immigrant population that has been deeply affected by the president’s deportation crackdowns, said that she was undecided, but that “people vote for me in South Florida, and I want to be there with them.”

Representative Juan Ciscomani, who is running in a competitive district in Arizona, had a ready excuse for his likely absence: He is scheduled to appear at a debate against his Democratic challenger on Sept. 10 that he said he needed to prioritize, he told The Bulwark.

He was hardly alone among Republicans in having something else to do that night.

The Texas confab is being billed as a chance for the party to raise money and highlight candidates ahead of an election cycle in which Republicans are struggling to present a persuasive message to voters and are bracing to lose control of the House and, possibly, the Senate.

It comes at a time when the president’s tariffs and the war in Iran remain deeply unpopular. And it lands amid growing Republican frustration that Mr. Trump’s political operation, MAGA Inc., which has a $400 million war chest, has so far stayed almost entirely on the sidelines in the party’s fight to keep control of Congress.

A former top House G.O.P. leadership aide, speaking on condition of anonymity to discuss private conversations, said that most Republican lawmakers viewed the week as better spent on the ground at home instead of inside a convention center in Dallas. The aide said he agreed with them strategically.

Speaker Mike Johnson, however, has been talking it up.

“The first ever midterm convention is something that’s going to be so exciting,” he said Tuesday, noting that he had been promoting it all summer as he traveled to 52 districts across 18 states. “We’re going to have a huge number of our members and candidates there.”

And a spokeswoman for the Republican National Committee, Natalie Baldassare, disputed that Republicans were skipping the event, but declined to provide a list of attendees.

“The outreach and enthusiasm to attend, participate and speak at this historic midterm convention has been incredible, from members of Congress to Republican candidates to everyday Americans,” Ms. Baldassare said. “This convention will provide the momentum needed to expand our majorities in November.”

A handful of at-risk Republicans were expected to attend. One was Representative Mike Lawler, a Republican from a competitive district north of New York City. He has sought to distance himself from the president and is running an ad that features the president calling him a “pain in the ass” for advocating the repeal of the cap on the state and local tax deduction.

Mr. Lawler said in an interview with MS NOW that he would be “introducing” two of his constituents, whose daughter, he said, was murdered by an undocumented immigrant.

Representative Rob Bresnahan, a vulnerable Republican from Pennsylvania, was billed as a speaker, according to plans shared with The New York Times, along with Senator Jon Husted of Ohio, who is fending off a strong challenge from former Senator Sherrod Brown. Former Representative Mike Rogers, who is running for Senate in the competitive state of Michigan, and Representative Derrick Van Orden of Wisconsin, a top target Democrats are hoping to unseat, are also set to speak.

But several Republicans in difficult races tried avoiding the question of whether they would attend.

Representative Mariannette Miller-Meeks of Iowa, who has been trailing her Democratic opponent in recent polls, belligerently refused to tell a reporter at the Capitol whether she would attend.

“Call my communications director,” she said angrily as she left the House floor.

(She did, however, tell The Bulwark that she would not be there.)

Representative Jen Kiggans, a vulnerable Virginia Republican, walked to evening votes on Tuesday night flanked by two aides who yelled at reporters to leave her alone. Ms. Kiggans declined to answer whether she planned to attend.

Others who said they were skipping the gathering: Representative Thomas Kean Jr. of New Jersey, the Republican who went missing from Congress for more than 100 votes before explaining that he had been hospitalized for depression; Representative Tom Barrett of Michigan, running in one of the most competitive districts in the country; and Senator Susan Collins of Maine, who routinely breaks with the president and tries to distance herself from an administration that Democrats are eager to tie her to.

Mr. Valadao said that Mr. Trump still had a part to play in the midterm elections, but that navigating around a deeply unpopular president and his policies was a challenge.

“He’s going to play a role with the base,” he said. “Getting the base motivated and showing up is important. There are spots where he can be helpful. There are spots where he can be less helpful. It’s a balance to try and figure out the best way to message and focus on the positives.”

Asked whether he expected help from the president in keeping his seat, Mr. Valadao said: “I don’t really bank on it.”

Olivia Diaz contributed reporting.