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.

Despite its apparent opportunism, Ataka’s one constant has been its vicious rhetoric against foreigners and minorities. Alfa Television, a station operated by Ataka, denounces the refugees as radical Islamists and scroungers who will only bring violence and deeper poverty to Bulgaria.