Republicans running for the U.S. Senate and House gleefully seized on President Biden’s stumbles in Thursday’s debate, betting they could use his performance to drag down their Democratic opponents.
Many have spent months trying to tie their foes to the president. But even as Mr. Biden trails in swing-state polls, Democratic incumbents in those states have proved resilient against their Republican challengers, sometimes outrunning the president by a dozen percentage points in surveys.
Now Republicans are sensing an opening with voters by questioning how Democrats could stand behind a president whose halting and raspy performance on Thursday reignited questions about his age and acuity.
David McCormick, the Republican businessman trying to oust Senator Bob Casey in Pennsylvania, called out Mr. Casey’s support for Mr. Biden in a post on X.
“Bob Casey has said over and over that his ‘close friend’ Joe Biden, with whom he votes 98 percent of the time, is fit to be president,” Mr. McCormick wrote. “What we all saw last night proves Casey is lying.”
In Arizona, where the Trump acolyte Kari Lake is trying to stop Representative Ruben Gallego from picking up the seat held by Kyrsten Sinema, the Democrat turned independent who is not seeking re-election, Ms. Lake has tried repeatedly to compare Mr. Gallego to Mr. Biden. She has derided him as Mr. Biden’s “mini-me” and has said he is essentially the same as Mr. Biden, but 40 years younger.
A senior adviser to Ms. Lake’s campaign, who insisted on anonymity to discuss internal data, said the campaign had found that the closer it tied Mr. Gallego to Mr. Biden, the more skeptical independent voters grew of Mr. Gallego.
“Last night, the entire nation witnessed how unfit Joe Biden is,” Ms. Lake said in a statement. “As more voters discover that Ruben is nothing more than an extension of Joe, they will join our movement.”
Sam Brown, an Army veteran and Republican who is trailing Senator Jacky Rosen in polls of Nevada even as Mr. Trump is leading Mr. Biden there, wrote that during the debate, “Nevadans saw with their own eyes what a Joe Biden/Jacky Rosen ticket will look like in November, and it is disastrous.”
The National Republican Senatorial Committee, which works to elect Republicans to the Senate, hammered Senator Tammy Baldwin, a Wisconsin Democrat, for standing by Mr. Biden after initially being slow to do so.
“Tammy Baldwin is playing cleanup after dodging the president’s disturbing performance in last night’s debate,” said Tate Mitchell, a spokesman for the N.R.S.C. “The debate only sowed more doubt in his ability as president. Why can’t Tammy Baldwin admit that?”
Friday afternoon, the group released a video featuring clips of several Democrats voicing their support for the president. “Democrats 2024: Don’t believe your lying eyes,” it was captioned.
On the other side of the aisle, the Democratic Senatorial Campaign Committee downplayed the debate. “Senate campaigns are candidate vs. candidate battles, and Republicans have a roster of deeply flawed recruits,” David Bergstein, a D.S.C.C. spokesman, said in a statement.
Jeremy Hughes, a Republican strategist who works on Nevada races, said Republicans needed to keep working hard to overcome their Democratic opponents. They “must resist the urge to dance on Biden’s political grave,” he said. “They shouldn’t be measuring the drapes.”
“Of course” Mr. Biden is a drag on down-ballot Democrats, he said. Republicans should “pray that Joe Biden is so self-absorbed and narcissistic that he stays in the race,” Mr. Hughes said.
Privately, a handful of Democratic strategists across swing states expressed alarm about Mr. Biden’s performance and how it might weigh on their candidates’ chances. But some hoped that with campaigns and voters having long considered his age, the debate would not greatly affect down-ballot races.
Conor O’Callaghan, one of the Democratic front-runners vying to challenge Representative David Schweikert, a Republican, in a competitive Arizona House district, said he would stand by the president. But securing a Democratic majority in Congress is now even more vital, he said.
“That’s the only way we’ll be able to support President Biden’s agenda” Mr. O’Callaghan said. “And God forbid Trump wins, we will need a Dem-controlled Congress to keep him and Project 2025 in check.”
Others privately admitted to deep consternation. The worries were particularly acute in Montana, a deeply red state where Senator Jon Tester, a Democrat, has been navigating how to take credit for the Biden administration’s accomplishments while keeping the president himself at arm’s length.
A Democratic strategist in Montana who insisted on anonymity in order to discuss private conversations said that a dozen of the state’s Democrats had expressed fear that Republicans could use Mr. Biden’s stumbles to nationalize the race in a way that could damage Mr. Tester, who has survived past elections by leaning on his local roots.
Mr. Sheehy is aiming to do just that.
“After Joe Biden’s abysmal debate performance tonight, remember this, Montana — Jon Tester loves Joe Biden,” Mr. Sheehy posted on X. “Tester thinks Biden is ‘100 percent with it’ and votes with Biden 95 percent of the time!!”