Citing the Kosovo troubles, Mr. Botsan-Kharchenko, Moscow’s ambassador, told RT Balkan, a Russian state media operation, that “the goal of the West is to change the politics of Serbia” — a message that played to nationalist Serbs who see Russia as their defender and loathe the United States because of NATO’s 1999 bombing campaign during a war over Kosovo.

A recent survey by Demostat, a research group, found that only 3 percent of Serbs said they admired the United States and wanted good relations with it, in contrast to 22 percent who felt that way about Russia. At the same time, 32 percent favored the European Union and Scandinavian countries, indicating that support for Russia, though strong, lags behind that for the West overall.

And what the Russian ambassador presented as a Western plot to stir up trouble in Kosovo and unseat Mr. Vucic is seen as exactly the opposite by most experts and also by protesters.

Kosovo, said Cedomir Cupic, a political science professor at the University of Belgrade, “is already lost” because there is no realistic possibility of Serbia taking back and ruling more than a million restive ethnic Albanians. But for Russia, he said, the domestic passions it still generates are a godsend for Moscow — a “toothpick that it can always poke around to make the U.S. and Europe feel nervous.”

The violence in Kosovo has also provided a rare piece of good news for Mr. Vucic by playing to his strong suit as defender of Serb interests as he struggles to defuse the street protests.