Poland votes on Sunday in a tight runoff vote for the presidency. It is a largely ceremonial post, but its veto powers have made the election of vital importance to Prime Minister Donald Tusk and to his centrist government, which is still trying to reverse the legacy of its hard-right predecessor.

The most populous economic and military power on the European Union’s eastern flank, Poland plays an important role as a bellwether. Its election is being closely watched as a sign of whether populist nationalism is a rising or receding force on the continent.

The two candidates represent starkly different political outlooks: Rafal Trzaskowski, 53, is the liberal, multilingual mayor of Warsaw and the son of a prominent jazz musician. Karol Nawrocki, 42, is a nationalist historian and former boxer.

Mr. Trzaskowski is supported by Mr. Tusk’s party, Civic Platform, and, if elected, would work closely with his government.

Mr. Nawrocki, a political novice, is hostile to Mr. Tusk and is backed by Poland’s previous governing party, Law and Justice.

If elected, Mr. Nawrocki would most likely use the powers of the presidency to block Mr. Tusk’s agenda, as the departing president, Andrzej Duda, has. He was able to obstruct it by vetoing legislation passed by Parliament or referring it for review by courts stacked with Law and Justice appointees.