In Kansas, Senator Roger Marshall, a vulnerable Republican, baselessly said his Democratic opponent “probably is grooming” children.

In Texas, Republicans have cast James Talarico, the Democratic Senate nominee, as a “freak” and insinuated he is gay, secretly a vegan, and obsessed with transgender children because he made statements supporting them in the past.

And in Michigan, Republicans have targeted the Democratic Senate nominee, Dr. Abdul El-Sayed, with Islamophobic attacks. Mr. Trump has described Dr. El-Sayed as a “vicious, violent jihadist.”

Facing a backlash over rising prices and working to overcome the president’s deep unpopularity, Republicans across the country have tarred their opponents with false, inflammatory and deeply personal attacks in the final stretch of the midterm election.

In many cases, they are following the lead of a president who has repeatedly rewritten the rules of acceptable discourse in American politics. Many of the attacks have drawn little pushback from within the party, but some Republicans say they are concerned by the trend.

“It’s awful,” said former Senator Jeff Flake, an Arizona Republican and critic of Mr. Trump. Mr. Flake, who is backing candidates in both parties in the midterms, said Republicans had gone down a darker road in their messaging: “We’ve gotten progressively more crass and crude and vulgar. But this seems to be a new level.”

The strategy risks turning off some moderate voters who see the rhetoric as over-the-top or offensive, political observers said. But it could be effective at energizing the Republican base. Democrats, who have widely condemned the Republicans’ attacks, worry the lines will land with some voters.

Democrats themselves have at times used strikingly personal attacks, often accusing Republicans of corruption. And some Democrats have imitated the president’s crude approach: Gov. Gavin Newsom’s office, for example, made social-media posts this year suggesting that various conservatives were gay.

But in the midterms the Republican attacks stand out for their viciousness.

American political campaigns have long been awash in negative and dubious attacks by both parties — in debate lines, TV commercials and stump speeches — but this bitter, hard-fought cycle may set a new standard. Some political scientists said this election season had devolved into the most negative, dark campaign in modern American history.

“I think it’s the worst,” said Christina Greer, a political science professor at Fordham University. She attributed the tone to candidates’ desire to package their message for social media, and to Republicans’ mirroring of Mr. Trump: “He’s opened a portal to the degradation of our electoral system.”

In Texas, Vice President JD Vance suggested at a rally that Mr. Talarico was gay, drawing gasps from the crowd. “A guy that age who is so obsessed and focused on transgender children, I don’t know about you, but I never want to see that guy’s web browser history,” Mr. Vance added, referring to Mr. Talarico’s past statements in support of transgender youth.

Mr. Talarico has a girlfriend. The attack was part of a broad effort by Republicans to portray Mr. Talarico as extreme. Mr. Trump has taken to referring to Mr. Talarico as “Talafreako.”

Vice President JD Vance at a campaign rally in Tyler, Texas.Credit…Pool photo by Saul Loeb

A spokeswoman for the National Republican Senatorial Committee suggested that Democrats had brought the attacks upon themselves. In a statement, the spokeswoman, Bernadette Breslin, claimed that Dr. El-Sayed had “pledged support” for a terrorist regime and said that “if Talarico didn’t want to be called anti-meat, he shouldn’t have announced his campaign would go vegan.”

Dr. El-Sayed has said he is loyal to the U.S. Constitution. Mr. Talarico has said he is not a vegan and that his campaign “basically” runs on barbecue.

The White House did not immediately respond to a request for comment.

Mr. Flake said the coarseness of some Republican attacks stemmed from a mix of desperation in the party and a mimicking of Mr. Trump. But he said he found a “silver lining” in the backlash some of Mr. Trump’s “imitators” had experienced — including in Republican criticism of a headline-grabbing attack ad in the Ohio governor’s race.

That ad came from the campaign of Vivek Ramaswamy, the Republican nominee for governor. The commercial featured the mother of his Democratic opponent, Dr. Amy Acton, who says she has not spoken to her mother since she was 12, after her stepfather was charged with raping her. The sitting governor of Ohio, Mike DeWine, a Republican, called the ad “deeply, deeply troubling,” even as Mr. Trump and Mr. Vance defended it.

Former President Barack Obama, appearing at a rally in Ohio this month, said he was baffled that “the standards of behavior have gotten so low that somebody can run an ad like that.”

Some Republicans have criticized Vivek Ramaswamy, the Republican candidate for Ohio governor, for his ad that featured the estranged mother of his opponent, Dr. Amy Acton.Credit…Maddie McGarvey for The New York Times

In Maine, Senator Susan Collins, a Republican who built a brand on civility, denounced a radio ad from the top super PAC for Senate Republicans that cast her Democratic opponent, Troy Jackson, as a “freak” for having two children with his second cousin. Ms. Collins called for the ad to be taken down.

But Ms. Collins, who has deplored Mr. Jackson’s efforts to cast her as corrupt, herself commented on Mr. Jackson’s family in the summer, telling Politico: “I’m not going to bring up the fact that the mother of his two children is his second cousin.”

Other Republican attacks have gone largely unchallenged inside the party. After Mr. Marshall suggested his Democratic opponent, Adam Hamilton, was a “groomer” of children, there was no significant Republican criticism. And there was no Republican uproar after Mr. Trump used an anti-trans slur to refer to the Democratic Senate nominee in South Carolina, Dr. Annie Andrews, calling her “Tranny Annie.” (Dr. Andrews is not transgender.)

In Kansas, Senator Roger Marshall, a vulnerable Republican, has baselessly said his Democratic opponent “probably is grooming” children.Credit…Eric Lee for The New York Times

A spokesman for Mr. Hamilton, Tyson Brody, said in a statement that Mr. Marshall was “resorting to increasingly unhinged and inexplicable smears.” Dr. Andrews released an ad this month in which she said she did not support transgender women playing in women’s sports or gender transition surgeries for minors. (In 2022 she had spoken out against a trans sports ban in South Carolina.)

On the campaign trail in Michigan, Mike Rogers, the Republican candidate for Senate, has often brought supporters who have questioned Dr. El-Sayed’s loyalty to America.

At a Rogers rally in Plainwell, Mich., on Thursday, Senator Tim Sheehy, Republican of Montana, told the crowd, “You better be afraid,” suggesting that supporting Dr. El-Sayed would be a step toward Shariah law in Michigan. “We have a radical Islamist on the other side of this ticket who hates this country.”

Asked later about Mr. Sheehy’s comments, Mr. Rogers told reporters, “You haven’t heard me say any of those things.”

Senator Tim Sheehy, Republican of Montana, in Washington. This week, he told a crowd in Michigan, “You better be afraid,” suggesting that supporting Dr. Abdul El-Sayed would be a step toward Shariah law.Credit…Al Drago/Reuters

Dr. El-Sayed said at a rally on Sunday that Mr. Rogers’s “whole campaign is to point at me and my name and my faith.”

“But my name and my faith are not the reason you can’t afford your gas,” Dr. El-Sayed said. “My name and my faith are not the reason you can’t afford your groceries.”

Islamophobic attacks have long been a feature of American politics. But in the past, candidates in both parties sought to push back. In 2008, the Republican nominee for president, Senator John McCain, defended his opponent, Barack Obama, when a woman at a rally said she did not trust him because “he’s an Arab.” Mr. McCain replied, “No, ma’am, he’s a decent family man.”

This year, Democrats say they fear that Islamophobic attacks on Dr. El-Sayed have resonated with some voters and are hurting his chances in a race that is critical in the battle for Senate control.

“I won’t say it’s the worst and most horrific ever,” Mike McGuinness, the Democratic mayor of Pontiac, Mich., said of the Republican attacks on Dr. El-Sayed’s faith and background. “But it is definitely a Hall of Fame contender.”

The Rev. Horace Sheffield III of New Destiny Christian Fellowship in Detroit said the rhetoric in the campaign felt to him as if it came from long ago in American history.

“This tenor and tone that we find ourselves in now is very dangerous,” said Mr. Sheffield, who supports Dr. El-Sayed and whose daughter is Mayor Mary Sheffield of Detroit. “And it’s all based on prejudice.”

Cayce Myers, a professor of political communication at Virginia Tech who studies negative ads, said the race in Michigan was a “test case” for how inflammatory a message could be and still help, rather than hurt, a candidate in 2026.

Negative ads typically are the most motivating, he said, and Republicans in Michigan and elsewhere appeared to be betting that their approach would help activate base voters more than it would alienate voters in the middle.

But, Professor Myers added, Republicans are “pushing up against the limit of what voters and the electorate have an appetite for.”