The local leader of Ataka, a pugnacious, far-right party, Mr. Bozhinov lost his seat in the town 

council at the last municipal elections in 2011 but now sees his fortunes rising thanks to public alarm over an influx of Syrian refugees across the nearby frontier. 

Membership of the local branch of Ataka, he said, had surged in recent weeks as “people come up to me in the street and tell me that our party was right.” Ataka, which means attack, champions “Bulgaria for Bulgarians” and has denounced Syrian refugees as terrorists whom Bulgaria, the European Union’s poorest nation, must expel. An Ataka member of Parliament has reviled them as “terrible, despicable primates.” 

With populist, anti-immigrant parties gathering momentum across much of Europe, Ataka stands out as a particularly shrill and, its critics say, sinister political force — an example of how easily opportunistic groups can stoke public fears while improving their own fortunes. 

The influx of Syrian refugees has sown divisions across the European Union as the refugees add burdens on governments still struggling to emerge from years of recession. But Bulgaria is perhaps the most fragile of all the European Union’s 28 members. Modest as the numbers of refugees are here, the entry of nearly 6,500 Syrians this year has overwhelmed the deeply unpopular coalition government and added a volatile element to the nation’s already unstable politics. 

The arrival of the refugees and public fury over the stabbing of a young Bulgarian woman by an Algerian asylum seeker “has opened the floodgates” for far-right nationalists, said Daniel Smilov of the Center for Liberal Strategies, a policy research group in Sofia, the capital. “They see this as their big chance.” 

No matter how inflammatory its message or small its numbers, Ataka has had outsize leverage since inconclusive parliamentary elections in May left it critical to the survival of Bulgaria’s new socialist-led government, which has been besieged for months by protesters demanding its resignation over complaints of cronyism and corruption. 

Ataka’s denunciations of foreigners and local minorities like the Roma, or Gypsies, along with its demands for the return of “lost” Bulgarian territory, have pushed it to the forefront of a chauvinistic, nationalist trend gaining ground in several former Communist countries of Eastern and Central Europe. 

Like many populist parties in Western Europe, Ataka mixes right-wing calls for law and order and restrictions on immigration with economic policies that veer sharply to the left. But more than nationalist groups in richer countries to the West, Ataka is “virulently racist and anti-Semitic,” said Krassimir Kanev, head of the Bulgarian Helsinki Committee, an advocacy group working to improve the often dire conditions in government-run camps now housing the refugees. 

The party was more comparable to the neo-fascist Golden Dawn movement in Greece, he said, but conceded, “Ataka’s strategy works.” Yet while Greece’s government has cracked down on Golden Dawn, in Bulgaria, Ataka is in effect a government ally. “They have made the political debate harsher and harsher,” Mr. Kanev added, as Ataka and like-minded parties have defied predictions that xenophobic extremism would fade away as Europe’s poorer nations, particularly former Communist ones, became integrated into the European Union. 

Hungary’s far-right Jobbik, the third-biggest party in Parliament, stirred outrage in early November when it unveiled a statue in Budapest of a Nazi collaborator, Miklos Horthy, the country’s fascist leader during World War II. Later in November, in neighboring Slovakia, Marian Kotleba, a fiery nationalist notorious for incendiary comments about the Roma, won election as a regional governor.

Over the years, Ataka has allied itself with mainstream parties on both the right and the left, and even with an ethnic Turkish party that it previously wanted banned. 

Though Ataka won only about 7 percent of the vote and 23 seats in Bulgaria’s 240-member Parliament in the May election, this political shape-shifting has helped make it a kingmaker in the finely balanced assembly. It has used its ever-changing alliances to push through Parliament a ban on land sales to foreigners, in violation of Bulgaria’s commitments to the European Union, and also to prod the authorities to start building a high fence along part of the border with Turkey to keep out the Syrians.