Karol Nawrocki, a historian and former boxer, will be Poland’s next president. Nationalist forces in the United States and Europe had been rooting for him: Mr. Nawrocki was supported by President Trump, Prime Minister Viktor Orban of Hungary and George Simion, a right-wing politician in Romania who recently lost a presidential election.

Poles awakened Monday to a clear albeit close result that returns a nationalist politician to the presidency who has pledged to hinder the centrist, pro-EU government for the remainder of its term.

We won!” exclaimed Rafał Trzaskowski, Warsaw’s liberal mayor, as initial exit polls showed him edging ahead of his opponent, Karol Nawrocki, 50.3 percent to 49.7 percent, in Poland’s presidential election on June 1,