It last expanded in 2013, when Croatia joined.

Some critics have criticized Mr. Vucic’s move as a ploy to solidify his power, as his center-right Serbian Progressive Party already has a comfortable majority in Parliament.

Mr. Vucic was once an arch-nationalist and a close ally of the Serbian strongman Slobodan Milosevic, who died in 2006 while facing international war crimes charges relating to the Balkan conflicts of the 1990s. Mr. Vucic, who turns 45 on Saturday, has since cast himself as a pro-Western liberal who favors close relations with Europe and the United States. But he has also come under some criticism, fairly or not, for being thin-skinned and for discouraging dissenting voices, including in the news media.

Srdjan Bogosavljevic, a consultant at the Ipsos polling company, said the call for early elections was a canny move that would allow Mr. Vucic to consolidate his power and to proceed with plans to shake up the country’s bloated public sector and to overhaul its laws. “Serbia is still a hangover from the former Yugoslavia, and our legal system needs updating,” Mr. Bogosavljevic said. “No one here sees Europe any longer as a dream, as they once did. But they can’t see any better alternative.”

Serbia, a relatively poor Balkan country, has been seeking to modernize its economy and to improve its relations with its neighbors as a prelude to joining the European Union, the world’s largest trading bloc. But the country has a weak economy and double-digit unemployment, and Europe’s volatile migration crisis has been spilling across its borders. In late 2014, it agreed to a 1.2 billion euro ($1.3 billion) loan agreement with the International Monetary Fund, under which it is required to shore up its finances by, among other things, privatizing state industries.

At the same time, Serbia has been struggling to forge good relations with Kosovo, which declared independence from Serbia in 2008, amid ethnic tensions that had flared during the Balkan wars of the 1990s. Serbia and its ally, Russia, do not recognize Kosovo’s independence.