Liesl Hickey, a Republican strategist involved in several House races in swing states, said she was dismayed by a sudden exodus of independent voters in more diverse parts of the country.
âThey are really starting to pull away from Trump,â said Ms. Hickey, describing his soaring unpopularity with independents as entering âuncharted territory.â
Mr. Trumpâs erratic behavior last week after his poor performance in the first debate with Mrs. Clinton â attacking a former beauty pageant winner over her weight, and making an issue of the Clintonsâ marriage â has alarmed a number of Republican senators, including Mitch McConnell of Kentucky, the majority leader. Mr. McConnell expressed concern that Mr. Trump might not have bottomed out yet and could lose even more support among women, according to a Republican official who spoke on the condition of anonymity to recount a private conversation.
If the roller-coaster dip in Mr. Trumpâs standing has heightened anxieties among Republican officials and political operatives, a steady if unspectacular performance by his running mate, Gov. Mike Pence of Indiana, in the vice-presidential debate on Tuesday failed to quiet their nerves.
âTwo weeks ago I would have said Republicans would hold control of the Senate, but thereâs just so many seats up and nobody is getting separation,â said Neil Newhouse, a Republican pollster, referring to the number of the partyâs candidates still locked in tight races. âIt worries me that weâre this close to Election Day and youâre not seeing that separation, because it makes you wonder what kind of impact the top of the ticket has.â
Other Republicans are holding out hope that Mr. Trump can at least execute what some cheekily call a âlose-closeâ strategy: holding Mrs. Clinton to a narrow victory, and sparing other Republican candidates in the process.
Jay Bergman, a petroleum executive and Republican donor from Illinois, said his fellow contributors were no longer optimistic that Mr. Trump will win, and they have lowered their sights. âThey want the guy to make a credible showing,â he said. âTheyâre afraid that if Trump really screws up and looks bad, then down-ticket, there are going to be a lot more votes for Democrats.â
If Mrs. Clinton wins, putting Tim Kaine, as vice president, there to break a tie, Democrats would need four seats to take control of the Senate. Officials in both parties see Republican incumbents in Wisconsin and Illinois as likely to lose, so Democrats would need to just two more pickups to capture the majority if they retain the rest of their seats.
Republicans worry that Mr. Trumpâs difficulties in New Hampshire and Pennsylvania, where Republican incumbents are caught between their own base and moderate voters appalled by the partyâs nominee, could hand Democrats those decisive seats. Senator Kelly Ayotte, the Republican up for re-election in New Hampshire, demonstrated the vise she is in this week when she said at a debate that Mr. Trump would represent a good role model for children, only to recant a few hours later.
Sensing new opportunity, Democrats intend to redouble their efforts to tie Republican candidates to Mr. Trump in states and districts with large numbers of college-educated voters and minorities.
âI think itâs quite effective in New Hampshire, in suburban Philadelphia and in Nevada,â said Senator Chuck Schumer, a New York Democrat.
Compounding their difficulties, Republicans are also fending off a challenge to Senator Richard M. Burr in North Carolina, a state Mrs. Clinton is determined to win, and have also become just as worried about Senator Roy Bluntâs prospects in Missouri. Strategists in both parties who have seen internal polling say Mr. Blunt, whose seat initially seemed safe, is now trailing his Democratic challenger, Jason Kander, a deft campaigner who has been helped by Mrs. Clintonâs narrowing deficit in the state.
The good news for Senate Republicans, besides Mrs. Clintonâs own unpopularity and Mr. Trumpâs history of bouncing back from self-inflicted wounds, is that Democrats may need to pick up more than just two seats to seize the majority. In Nevada, which has the only Democratic-held Senate seat being aggressively fought over this year, strategists in both parties say Republicans have an advantage in the race to succeed Harry Reid, the minority leader.
Some Republicans doubt the party will take the step of completely abandoning Mr. Trump unless a landslide gap opens in the presidential race. In that event, Democrats intend to appeal to Mrs. Clinton to spend more of her time and money in areas where the partyâs congressional candidates are struggling.
In the House, where Republicans enjoy a 59-seat majority, the partyâs strategists still insist that Mr. Trumpâs effect has been limited; while his poll numbers have fallen since the first debate, he is not yet seen as so much of a drag on the ballot that he could send the partyâs other candidates to defeat.
House Democrats, however, finished polling 30 battleground districts last week â before the fallout from the first presidential debate â and concluded that Mr. Trump remained toxic for Republican congressional candidates. Geoff Garin, one of the Democratic pollsters who conducted the survey, said undecided or wavering voters tended to see Republicans as âputting party loyalty ahead of the country by supporting Trump.â
âCandidatesâ support for him and unwillingness to stand up to him becomes a black mark,â Mr. Garin said.
In a growing list of House races, Democrats are showing ads that link Republican lawmakers directly to Mr. Trump. A commercial in California brands Representative Jeff Denham as âDonald Trumpâs man in Washington.â An ad in Orlando, Fla., describes Representative John L. Mica as having âthe same harmful views on womenâ as Mr. Trump.
Kelly Ward, executive director of the Democratic Congressional Campaign Committee, said swing voters tended not to distinguish between lawmakers who vocally endorsed Mr. Trump and those who have stayed silent, neither supporting nor actively opposing his candidacy.
âTheyâre trying to live in this mushy middle, and I think thatâs where voters will hold them accountable,â Ms. Ward said. âThe separation Republicans think they will see from Donald Trump is just defied by history.â
A handful of Republicans have explicitly sought distance from Mr. Trump already. Robert J. Dold, a congressman from Illinois who has said he will not vote for Mr. Trump, began running a commercial this week that shows him switching off a television broadcast featuring clips of Mr. Trump and Mrs. Clinton. And veteran Republicans have urged Representative Scott Garrett of New Jersey, who is seeking re-election in a district Mr. Trump is behind in the polls, to portray himself as a check on Mrs. Clinton, though he has not yet done so.
Few have gone even as far as Mr. Dold, fearing backlash from Mr. Trumpâs ardent supporters â and perhaps from Mr. Trump himself, who has repeatedly attacked Republicans who have snubbed him. Indeed, at a fund-raising event outside Chicago last month, Mr. Trump noted in a biting aside that Senator Mark S. Kirk of Illinois, a Republican who opposes Mr. Trump, was on track to be defeated, according to Mr. Bergman, who attended the event.
If Mr. Trump fails to recover, Republicans still question whether Mrs. Clinton is capable of piling up enough of a victory margin to pull congressional Democrats into office along with her.
Mike Shields, president of the Congressional Leadership Fund, a âsuper PACâ that supports Republican House candidates, said Democrats were unlikely to win races on Mr. Trumpâs weakness alone.
âWe accepted that we had a challenging nominee,â Mr. Shields said. âBut in some districts where Trump is either down or has a very low approval rating, they are not able to take advantage of it.â
Mr. Shields added that Democrats have âhad to delude themselves that thereâs only one presidential candidate running.â