Hoping to end to an argument that has dragged on since the collapse of the Soviet Union, Moldovans began voting on Sunday to decide whether to enshrine in their Constitution an “irreversible” commitment to leave Russia’s orbit of influence and one day join the European Union.

Moldova, one of the poorest countries in Europe, is also holding a presidential election that includes the pro-Western incumbent, Maia Sandu, and 10 other candidates, many of whom favor closer relations with Russia.

An independent state next to Ukraine that emerged from the ruins of Soviet power in 1991, Moldova has been tugged between East and West for decades, swerving between leaders who want to align with Russia and others, like Ms. Sandu, whose government in June began membership negotiations with the European Union.

Entry into the bloc is still many years away, but it has been accelerated by alarm in Western capitals over Russia’s full-scale invasion of Ukraine, which began in February 2022, and a new determination to prevent Moscow from advancing into other former Soviet territories.

Three Baltic States that were once part of the Soviet empire — Estonia, Lithuania and Latvia — were admitted into the European Union and NATO 20 years ago, a shift that has anchored them firmly in the West and has helped curb Russian ambitions to return them to rule by Moscow.

In response to the war in Ukraine, the European Commission, the bloc’s executive arm, recommended in June 2022 that Moldova and Ukraine be granted “candidate status,” the first formal step in a process that normally lasts longer than a decade.