Donald Trump’s struggling candidacy has become a direct threat to Republican control of Congress, significantly increasing the likelihood that Democrats will take control of the Senate and cut substantially into the House Republican majority next year.

Trump’s string of inflammatory statements in the weeks since his nominating convention last month has sent him tumbling in nearly every state with a contested Senate race, raising Republican fears that their own demoralized voters will not show up to vote, independents will abandon the entire Republican ticket and energized Democrats will flock to the polls.

While Republicans anticipate that their down-ballot candidates will be able to outpace Trump’s share of the vote, national and local party officials and strategists are increasingly concerned that he is in danger of being so soundly defeated that even their best-prepared candidates will not be able to withstand the backlash to the top of the ticket.

“People are getting pretty nervous about our candidates because he’s in a death spiral here and nobody knows where the bottom is at,” said Sen. Lindsey Graham, R-S.C., who is close to many of his colleagues facing re-election.

Trump’s move this week to overhaul his campaign by hiring the head of a conservative website known for its incendiary writings on race and ethnic identity only heightened Republican concerns about Trump’s impact on moderate voters. If he devotes the rest of his campaign to a platform of hard-edge nationalism, strategists said, it could further turn off the suburban centrists Senate Republicans need in their column.

While there is still time for the dynamics of the campaign to shift, what worries Republicans most acutely is Trump’s eroding position in three states that alone could determine control of the Senate: Pennsylvania, New Hampshire and North Carolina. Hillary Clinton has staked out a clear advantage in each state in public polling, and private surveys by pro-Republican groups mirrors the trend, according to multiple officials tracking the campaigns.

If Trump cannot close the gap and at least limit his eventual deficit to 5 percentage points or less in those states, Republican strategists say, Sens. Patrick J. Toomey of Pennsylvania, Kelly Ayotte of New Hampshire and Richard M. Burr of North Carolina will find their seats in jeopardy.

“The question is, how far ahead of Trump can they run?” said Karl Rove, founder of American Crossroads, a super PAC involved in this year’s Senate campaigns.

Many leading Republicans have concluded that Trump is sure to lose, and that the party should turn its attention entirely to buttressing its most endangered senators and limiting Democratic gains in the House. But there is a divide over how soon the Republican National Committee should begin shifting money away from the presidential race and to the fight to retain Congress.

“We’re rapidly approaching the time where the RNC will have to think long and hard about investing its resources in Senate seats rather than continuing to help a presidential campaign that’s going nowhere,” said Josh Holmes, a Republican strategist deeply involved in Senate races.

But at a private meeting of the Republican Governors Association this week in Aspen, Colo., Haley Barbour, an influential former Mississippi governor and Republican chairman, urged the governors to rally behind Trump to improve the party’s prospects and said the party should continue to back him, at least until October, according to a Republican who was there.

Democrats would need to pick up four seats to gain control of the Senate if Clinton wins the presidency, and her vice president becomes the chamber’s tiebreaker vote. Officials in both parties believe the two Republican seats most likely to change hands are in Wisconsin and Illinois. And with the well-known former Sen. Evan Bayh of Indiana returning home to try to reclaim his Republican-held seat, Democrats enjoy an initial advantage there, too.

So with only one Democratic-controlled seat being aggressively contested — that of the Senate minority leader, Harry Reid of Nevada, who is retiring — it is critical for Republicans to bolster their defenses in Pennsylvania, New Hampshire and North Carolina, or they are sure to lose the Senate.

Few professionals in either party believe the Republicans’ 30-seat majority in the House is yet at risk, largely because so many districts are drawn to make them uncompetitive in general elections. But strategists increasingly believe that Democrats could pick up more than a dozen seats, with their prospects brightest in the urban and suburban areas where Trump is losing by double digits.

“We have to be concerned,” said Rep. Charlie Dent, R-Pa. “I don’t think you can assume anybody is safe if the top of ticket is struggling as bad as this one is.”

House Democrats have begun trying to make life more difficult for Republicans in districts near large cities, airing cable ads laying the foundation for a fall campaign accusing Republican incumbents of putting loyalty to the party, and to Trump, ahead of the country’s best interests.

Some outside groups have started doing the same to vulnerable Senate Republicans. “If she’s so independent, why is she still supporting Trump?” asks Independence USA, a super PAC underwritten by former Mayor Michael R. Bloomberg of New York, in a television ad attacking Ayotte set to begin running Friday in New Hampshire.

But Republicans like Ayotte, who says she is voting for Trump but not endorsing him, could have a difficult time winning if Trump’s admirers spurn her.

“If she loses any of them, that could be an issue,” said Jeb Bradley, a Republican state senator in New Hampshire and a former member of Congress who lost a bid to reclaim his seat in 2008 thanks in part to the Democratic wave that year.

Burr, who has endorsed Trump, and Toomey, who has not, face a similar dilemma. They must determine how to retain Trump’s ardent supporters without embracing him, handing Democrats easy fodder and repelling independents.

The situation is far more manageable in those states where Trump is more competitive. In Ohio, for example, Sen. Rob Portman is seen as running a solid campaign. But Portman also retained his lead in part because even during this recent stretch, Trump has fallen behind Clinton only by around 4 points in public polling.

Some Republicans believe that, in Democratic-leaning or evenly matched states, Republican candidates are better off abandoning Trump because the party’s most reliable voters are still likely to go to the polls and support every Republican on the ballot.

“I think offending half of Republican primary voters is less risky than angering every swing voter in a state where the Republican base is not enough to win,” said Mike Murphy, a Republican strategist.

But other Republican leaders say the party is handcuffed to Trump for the time being. If the party collectively disavowed his candidacy now, they say, it would cause his poll numbers to sink even further while infuriating his supporters. And the internecine intraparty battle likely to ensue could depress Republican turnout, dragging other candidates down with him.

So for the sake of the rest of the ticket, these party leaders feel they have little choice but to try to prop up Trump the best they can.

“It’s going to be like ‘Weekend at Bernie’s’ — you got to make him look alive, even if he’s not,” said Graham, who is not supporting Trump.

What is gnawing at Republicans is the possibility that Trump has so turned off typically Republican-leaning college-educated and white female voters that they will never give him a chance — and that, as a result, he cannot come close to Clinton in swing states. Such a situation would free Democrats, the presidency in hand, to use the final weeks of the campaign to direct their resources toward winning as many congressional seats as possible.

“If national Democratic money pours into the state, and I think it will, that could change the equation,” said Phil English, a former Republican congressman from Pennsylvania.

Pennsylvania is the state that has most alarmed Republicans since the conventions. Trump’s standing has collapsed in the vote-rich Philadelphia area, helping Toomey’s challenger, Katie McGinty, establish a narrow edge in public polls.

“I am worried about the Southeast,” Rob Gleason, the Pennsylvania Republican chairman, said of the Philadelphia area, though he said that west of the Susquehanna River, Trump would pile up “record-level” margins for a Republican.

Burr, in North Carolina, is facing a similar challenge. While Trump is expected to run strongly in rural areas, he is so toxic around Raleigh and Charlotte that party strategists working on state legislative races describe, with a mix of horror and wonder, Republican-leaning suburban seats where their standard-bearer is trailing by well over 10 points.

The problem is serious enough that party officials there, where Gov. Pat McCrory is also facing a difficult re-election, have begun discussing ways to appeal to Republican voters who are uneasy about Trump but willing to support other Republican candidates.

The good news, they say, is that there is no longer a party lever in the state, making ticket-splitting easier. But it is the presidential nominees who have typically driven party turnout — not candidates in congressional or statewide races.

“Republicans are going to have to improve their turnout at the grass-roots level in a way we haven’t had to do in years past, if we want to win the Senate race,” said Dee Stewart, a Republican strategist in North Carolina.

Burr appears to be alert to the need to insulate himself and better establish his own identity in the state: He is to begin airing TV ads next week.

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