By Jonathan Martin and Alexander Burns

New York Times

Donald Trump’s support has plunged across the swing-state map during the last 10 days, wiping out his political recovery from September and threatening to undo weeks of Republican gains in the battle for control of Congress.

For his party, Trump’s reversal in fortune comes at the worst possible moment: Having muted their criticism of Trump in hopes that he could at least run competitively through Election Day, Republicans must decide in the next few days, rather than weeks, whether to seek distance from his wobbly campaign.

Should Trump falter badly in his second debate with Hillary Clinton on Sunday in St. Louis, Republican congressional candidates may take it as a cue to flee openly from their nominee, said two senior Republicans involved at high levels of the campaign who spoke on the condition of anonymity because they were not authorized to discuss private party strategy.

Trump has already slipped perceptibly in public polls, trailing widely this week in Pennsylvania and by smaller margins in Florida and North Carolina — three states he cannot afford to lose. But private polling by both parties shows an even more precipitous drop, especially among independent voters, moderate Republicans and women, according to a dozen strategists from both parties who spoke on the condition of anonymity because the data was confidential.

Trump’s erratic behavior last week after his poor performance in the first debate with Clinton — attacking a former beauty pageant winner over her weight, and making an issue of the Clintons’ marriage — has alarmed a number of Republican senators, including Mitch McConnell of Kentucky, the majority leader. McConnell expressed concern that 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 Trump’s 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.

“Two weeks ago I would have said Republicans would hold control of the Senate, but there’s just so many seats up and nobody is getting separation,” said Neil Newhouse, a Republican pollster, referring to the number of the party’s candidates still locked in tight races. “It worries me that we’re this close to Election Day and you’re not seeing that separation, because it makes you wonder what kind of impact the top of the ticket has.”

Other Republicans are holding out hope that Trump can at least execute what some cheekily call a “lose-close” strategy: holding 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 Trump will win, and they have lowered their sights. “They want the guy to make a credible showing,” he said. “They’re afraid that if Trump really screws up and looks bad, then down-ticket, there are going to be a lot more votes for Democrats.”

If 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 Trump’s difficulties in New Hampshire and Pennsylvania, where Republican incumbents are caught between their own base and moderate voters appalled by the party’s nominee, could hand Democrats those decisive seats. Sen. 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 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 Trump in states and districts with large numbers of college-educated voters and minorities.

“I think it’s quite effective in New Hampshire, in suburban Philadelphia and in Nevada,” said Sen. Chuck Schumer, D-N.Y.

Compounding their difficulties, Republicans are also fending off a challenge to Sen. Richard M. Burr in North Carolina, a state Clinton is determined to win, and have also become just as worried about Sen. Roy Blunt’s prospects in Missouri. Strategists in both parties who have seen internal polling say Blunt, whose seat initially seemed safe, is now trailing his Democratic challenger, Jason Kander, a deft campaigner who has been helped by Clinton’s narrowing deficit in the state.

The good news for Senate Republicans, besides Clinton’s own unpopularity and Trump’s 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 Trump unless a landslide gap opens in the presidential race. In that event, Democrats intend to appeal to Clinton to spend more of her time and money in areas where the party’s congressional candidates are struggling.

In the House, where Republicans enjoy a 59-seat majority, the party’s strategists still insist that Trump’s 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 party’s other candidates to defeat.

House Democrats, however, finished polling 30 battleground districts last week — before the fallout from the first presidential debate — and concluded that 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 “putting party loyalty ahead of the country by supporting Trump.”

“Candidates’ support for him and unwillingness to stand up to him becomes a black mark,” Garin said.

In a growing list of House races, Democrats are showing ads that link Republican lawmakers directly to Trump. A commercial in California brands Rep. Jeff Denham as “Donald Trump’s man in Washington.” An ad in Orlando, Florida, describes Rep. John L. Mica as having “the same harmful views on women” as Trump.

If Trump fails to recover, Republicans still question whether 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 super PAC that supports Republican House candidates, said Democrats were unlikely to win races on Trump’s weakness alone.

“We accepted that we had a challenging nominee,” Shields said. “But in some districts where Trump is either down or has a very low approval rating, they are not able to take advantage of it.”

Shields added that Democrats have “had to delude themselves that there’s only one presidential candidate running.”

© 2016 The New York Times Company