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.

Nationalist opposition candidate Karol Nawrocki narrowly won Poland's presidential election, results showed Monday, delivering a major blow to the centrist government's efforts to cement Warsaw's pro-European orientation.