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.

Mr. Nawrocki was backed by Poland’s previous governing party, Law and Justice, and is hostile to Poland’s central government and Prime Minister Donald Tusk, a veteran centrist.

His narrow but decisive win adds momentum to Europe’s right-wing populist movement and is likely to deepen Poland’s deep and acrimonious political divisions.

A brash political novice

Mr. Nawrocki, 42, is new to politics: He is a prominent conservative historian who once worked as a hotel security guard, as well as a boxer.

He has positioned himself as part of an unstoppable global right-wing populist movement and has tried to project a tough-guy, family-values persona. He posts often on social media about his young family and his Catholic faith and has also shared images of himself in the gym, boxing and firing guns.

His wife, Marta Nawrocka, is a civil servant. She has been a visible part of his campaign, as have their children: Daniel, Antoni and Katarzyna.