Republicans won the closely watched special election for a House seat Tuesday in North Carolina, but the close result is a fresh sign that the party risks losing the bigger war in 2020 for the suburban vote, the same bloc that propelled Democrats to take control of the House last year.

President Trump took a victory lap and Republican officials breathed a sigh of relief Wednesday after Dan Bishop squeezed out a 51% to 49% win over Democrat Dan McCready in a district that Mr. Trump carried by 12 percentage points over Democrat Hillary Clinton in 2016. At stake was an open seat that Republicans have held since the Kennedy administration.

Mr. Bishop and Republican-aligned groups invested $6 million in the race and brought Mr. Trump and Vice President Mike Pence to North Carolina to rally voters.

But pollster John Couvillon said the results out of southeast North Carolina fit a broader post-2016 trend in which Republicans have ridden to victory in special elections with the support of rural voters while hemorrhaging support in the formerly GOP-leaning suburbs.

It’s not a good trade-off in presidential election years, when a far larger electorate is in play, political analysts say. Defecting Republicans in suburban areas could change the calculus in a number of key states.

“What that means at a macro level is it definitely puts Arizona in play. It also means that Donald Trump is likely going to have to spend some [campaign] funds in Georgia, Texas and North Carolina,” Mr. Couvillon said. “Even though I think he will ultimately win those states, the fact that you have suburbs and urban areas where white-collar professionals are deserting the Republican Party, it will have a direct impact on the presidential election in those states.”

David Wasserman of the Cook Political Report told MSNBC Wednesday, “This is a five-alarm fire for Trump in the suburbs.” If the voting trends from Tuesday hold for 2020, he said, Mr. Trump would lose the state to a Democratic challenger.

Those four states also will have high-profile Senate races that could determine whether Republicans keep their slim 53-47 seat majority.

With Tuesday’s victory, Republicans now need to pick up 19 seats in the House to claim the majority, but a string of retirements could expand the playing field for Democrats. Republicans will have a hard time taking control of the lower chamber if they continue to lose support in the suburbs.

Kyle Kondik, of the University of Virginia’s Center for Politics, said the good news for Republicans is they most likely won’t have to spend heavily in North Carolina’s 9th Congressional District now that Mr. Bishop, a conservative state senator from the Charlotte suburbs, captured the seat.

“With Bishop in place, the district probably isn’t a major Democratic target next year,” Mr. Kondik said. “Trump may have helped with his late rally, but if [Mr. Trump] weren’t so unpopular overall, this race may not have been such an expensive slog for Republicans in the first place.”

Vulnerabilities for Democrats

The special election in North Carolina also exposed Democrats’ continued weakness in small towns and rural areas, which have provided a reliable bulwark for Mr. Trump and other Republicans, making it that much more difficult for even moderate Democrats to win in red states.

“From a Democratic perspective, it means the epicenter of your base is becoming more urban and suburban, and they continue to hemorrhage support in the rural areas,” Mr. Couvillon said. “In practical terms, that means if you are running statewide in Oklahoma, Kentucky or West Virginia, or rural Minnesota or rural Iowa, your life is getting tougher as well.”

House Democratic Caucus Chair Hakeem S. Jeffries argued that the special election in North Carolina should have never been so close and said House Democrats will have the chance to flip dozens of seats in less Trump-friendly districts next year.

“They squeaked by, and I don’t think any Republican in the House who’s concerned about his or her electoral prospects can feel better about the situation,” the New York Democrat said on Capitol Hill.

Meanwhile, Mr. Trump, who headlined an election eve rally with Mr. Bishop, expressed no doubts about what happened at the ballot box. He told reporters at the White House that the Bishop victory was “incredible” and a “tremendous win for the Republican Party.”

“The media thought he was going to lose,” Mr. Trump said. “They were all set to have a big celebration with their partners from the Democrat Party.”

In his postelection analysis, Mr. Wasserman said the outcome in North Carolina shows “the political environment hasn’t gotten much better for Republicans since the 2018 midterms” and that “many Trump voters aren’t jazzed about showing up for down-ballot GOP politicians when Trump’s not on the ballot.”

“The silver lining for red-seat Republicans: Trump will be back on the ballot next fall,” Mr. Wasserman said.

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