‘We just barely turned blue’: Bernie Sanders is getting stronger and some Democrats are getting worried.

Ever since Donald Trump won, Democrats have surged in the suburbs.

From Delaware and Chester Counties to the Lehigh Valley to bedroom communities outside Pittsburgh, and in suburbs of Detroit, Milwaukee, and Washington, Democrats have racked up wins in previously hostile territory, helped by affluent, college educated voters who have recoiled from the president.

But now, as self-described democratic socialist Sen. Bernie Sanders roars ahead in the presidential primary, winning Nevada’s caucuses Saturday to establish himself as the clear Democratic front-runner, some party officials in Pennsylvania and New Jersey worry that if he becomes the nominee, he could cost them hard-won gains.

Sanders argues his revolutionary zeal can energize previously disengaged voters among the young, people of color, and the white working class. But that approach runs counter to the get-things-done messages that helped moderate Democrats such as Chester County’s Chrissy Houlahan and South Jersey’s Andy Kim flip 40 Republican congressional seats in 2018 to take control of the U.S. House. Moderate approaches have also driven wins in state legislative, county commissioner, and even prothonotary races.

“Right now Chester County tends to be more in the moderate position,” said Dick Bingham, chairman of the county’s Democratic Party. "We just barely turned blue, so we’re still very much in that kind of middle of the political spectrum range.”

Republicans still outnumber Democrats there, so independents and GOP crossover voters have made the difference in Democratic victories, Bingham said.

Bingham, like other Democratic leaders interviewed, didn’t directly criticize Sanders, and predicted Democrats will unite around their party’s eventual nominee, no matter who it is. But party leaders did openly worry the Vermont senator could repel swing voters who can tilt tight races for Congress and the state legislature.

“Montgomery County is home to many moderate Republicans, and I hear from them with some regularity that they are interested in having a different president," said Val Arkoosh, a Democratic county commissioner there. "But they do have concerns over some of the more deeply progressive policies that are being espoused by Senator Sanders, so I think it could affect turnout from people who wouldn’t necessarily always vote Democratic.”

Endorsements from swing-district lawmakers in the region tell the story: They have all sided with centrists in the primary.

Houlahan and Rep. Conor Lamb, from outside Pittsburgh, support Joe Biden. Kim, of Burlington County, backs Pete Buttigieg. Farther north in New Jersey, Rep. Tom Malinowski endorsed Biden, and Rep. Mikie Sherrill sided with Mike Bloomberg. All of them won GOP-held battlegrounds in 2018, and several could face difficult fights to hang on to those seats.

Lamb forcefully criticized Sanders’ plan to ban fracking in a recent tweet aimed at the candidate and one of his top supporters, Rep. Alexandria Ocasio-Cortez (D., N.Y.).

“In the most important election of our lives, Sen. @BernieSanders & Rep. @AOC are celebrating a bill that would eliminate thousands of good union jobs in #PA17 & across PA — a state we need to win,” Lamb wrote Feb. 14. “What are we doing? We are Democrats. Jobs come first.”

Other Democrats in swing seats elsewhere in the country have been even more explicit in criticizing Sanders and worrying he could cost Democrats the House. On Thursday, another New Jerseyan in a competitive district, Rep. Josh Gottheimer, endorsed Bloomberg and specifically cited the down-ballot impact of the party’s presidential nominee.

"Democrats need a candidate at the top of the ticket who can win,” Gottheimer said in a statement that denounced “an all-or-nothing extremist approach to governing.”

“With so much at stake for Democrats up and down the ballot, we must unite behind a candidate who can appeal to voters across the spectrum,” he added.

No House member who flipped a Republican district has endorsed Sanders. But one of his national cochairs, Rep. Ro Khanna, of California, grew up in Bucks County and argued that Sanders’ message can win in Pennsylvania.

“Sanders’ message of health care for all, a living wage, and child care will resonate with many in Bucks County who want a better life for the working class and middle class,” Khanna said. “Sanders will be able to get record turnout. And members of Congress can tailor their message to fit their district. No one needs to run as a Sanders clone.”

Democrats, after all, saw that suburban gains weren’t enough to beat Trump in 2016, not as other parts of the state swung away from the party.

So far, Sanders’ message is working in the primary.

He raced to a virtual tie for first place in Iowa and an outright win in New Hampshire, the first two states to vote, before dominating in Nevada on Saturday. He has surged to the top of national polls. Numerous surveys show him running ahead of or close to Trump and increasing his support among nonwhite voters, critical in the Democratic primary. Young voters overwhelmingly favor him.

And after Trump’s win in 2016, supporters say voters are hungering for an alternative president who colors outside the normal political lines.

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But many vulnerable Democrats are still wary of being tied to Sanders.

Kathy Harrington, first vice chair of the Lehigh County Democratic Party, said one local official in a tough 2020 race has said that he’d avoid carrying Sanders literature when knocking on doors in conservative areas.

Harrington’s son, who is 29, is “in love with Bernie Sanders" and believes he’s the only person who’s going to be able to win. But, she added, “I’m in the car with someone right now who said she wouldn’t vote for Bernie Sanders even though she’s a Democrat. She’s making a gag noise right now.”

A national NBC/Wall Street Journal poll released Tuesday captured the divide. More registered voters were “enthusiastic” about Sanders, 16%, than about any other Democrat. But 44% were “very uncomfortable” with him, also the highest for any Democrat.

And while Sanders had a double-digit national lead over his Democratic rivals, the same survey found that 67% of voters had reservations or felt “very uncomfortable” with a socialist candidate, the worst result of any attribute tested. Sanders, an independent, identifies as a democratic socialist.

In Pennsylvania, Sanders is viewed unfavorably by 52% of registered voters, the worst of any Democratic candidate, according to a Quinnipiac University poll released Thursday (though Biden was close, at 51%).

The anxiety among center-left Democrats in many ways mirrors the tension swing-district Republicans felt in 2016, when Trump defied party insiders and became the GOP nominee.

Republicans in closely divided districts walked a tightrope. Some said they would support their party’s “nominee.” Others vowed to write in alternatives. Pennsylvania Sen. Pat Toomey waited until hours before the polls closed to announce he would, indeed, vote for Trump.

Despite predictions of doom, however, Trump won, and so did the swing-district Republicans around Philadelphia — though two years later most of them would retire or be wiped out.

Sanders supporters say the best path to victory is to generate a growing base of voters who can match the enthusiasm Trump draws. While swing-district Democrats emphasize cooperation and practicality, Sanders promises sweeping changes to health care and major parts of the American economy.

“In order to beat Donald Trump, Democrats will need to inspire a mass movement of working-class voters from all backgrounds and all corners of this nation," said Rosemary Boeglin, a Sanders campaign spokesperson. "His ability to expand the electorate by bringing in people who have been left out of the political process will drive the high voter turnout we need in the general election ... [and] will reverberate to the benefit of progressive candidates up and down the ballot.”

But as Trump has also demonstrated, rousing supporters’ passions can also fire up opponents and turn off swing voters. Moderates who dislike Trump won’t suddenly return to the GOP, said Josh Novotney, a Republican lobbyist in Philadelphia.

But Novotney said they might stay home if Sanders is atop the Democratic ticket: “It’s not too difficult to brand someone a socialist when they put it next to their name.”