A candidate for the resurgent far-right party Alternative for Germany has won a district administrator seat, the first time since the party was founded a decade ago that it has captured such a prominent regional post.

The candidate, Robert Sesselmann, won 52.8 percent of the vote, beating out the incumbent, Jürgen Köpper of the conservative Christian Democratic Union, who received 47.2 percent. The result has added to concerns from other parties that have struggled to respond to the rise of the AfD, as the party is known in Germany.

Although the post that went to the AfD is comparable to that of a mayor of a medium-sized German town, the victory resonated throughout the country, thanks to two breakthroughs: Although the party has a presence in Parliament, the vote will give it bureaucratic authority over an area for the first time, and it has never had the support of a majority of a district’s voters before.

“The result of the district council election in Sonneberg is disturbing,” Ricarda Lang, the co-chair of the federal Green party, wrote on Twitter. “And it is a warning to all democratic forces: Now, at the latest, is the time when — regardless of all disputes on issues — all democratic forces must defend democracy together.”

The victory on Sunday in the tiny district of Sonneberg, in the southern tip of the state of Thuringia, an emerging stronghold for the AfD in the former East Germany, came after a runoff was called when none of the candidates reached more than 50 percent in the regular election on June 11.

Now, Mr. Sesselmann will oversee an administration that is responsible for many aspects shaping the lives of the district’s residents, including infrastructure, employment offices and housing for refugees.

Christian Herrgott, the head of the Thuringia chapter of the C.D.U., said in a statement: “The election result makes it clear that we all have the task of finding solutions to this type of protest against Berlin.”

The AfD’s victory was especially troubling for its mainstream counterparts at least in part because the race in Sonneberg focused on criticism of federal politics, mostly eschewing the usual issues — such as parking, garbage or local finances — that are the bread and butter of district issues.

It is also the first concrete sign of success amid a general surge in the party’s popularity. Recent polls show that if elections were held today, the AfD would win about 20 percent of the vote nationwide, second only to the C.D.U., one of Germany’s major parties. The same polls indicate that the Social Democrats, who run the governing coalition, are lagging slightly behind the AfD.

“The real possibilities of a district administrator in a district with 54,000 inhabitants are limited, but this election victory gives the AfD a central position to attack state and federal politics,” Matthias Quent, an expert on far-right extremism, told the RND news outlet.

According to Germany’s domestic intelligence agency, roughly 10,000 of the AfD’s 28,500 members are extremists. Björn Höcke, who runs the Thuringia chapter of the party, is being investigated by a state prosecutor in Saxony-Anhalt for using a forbidden Nazi slogan during a campaign speech in 2021, but he is also one of the most popular figures among the party’s backers.

At least some of the AfD’s renewed support has been widely seen as a reaction to the government in Berlin: The country is struggling to transform its economy, introduce unpopular changes to energy policies and deal with migration at a level not seen since 2015 and 2016, when more than a million people entered the country. It was that final issue that helped the AfD win seats in Parliament.

As was the case in previous municipal and regional elections in which the AfD was in a position to win, mainstream parties rallied around a single candidate in Sonneberg and urged their voters to back Mr. Köpper.

“That was just the beginning,” Tino Chrupalla, the co-leader of the AfD’s national operation, wrote on Twitter. “We are winning over majorities with our policies for the interests of the citizens. This is how we will achieve a turnaround for the better for Germany.” Mr. Höcke predicted a “political earthquake” in coming district and state elections.

Bodo Ramelow, the far-left governor of Thuringia, attributed the result to the fault line that persists, more than three decades after reunification, between east and west. The AfD has found most of its support in parts of the former East Germany, where its anti-migrant stance has taken hold in a region where the economy has never reached parity with the former West.

“I think we have to redefine the spirit of German unity,” Mr. Ramelow told the public broadcaster ZDF after the results were announced on Sunday night, “that we bring the East Germans along with us and don’t create the feeling that they are laughed at or just talked about.”

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