In a setback for Europe’s surging nationalist forces, Nicusor Dan, a centrist mayor and former mathematics professor, on Sunday won the presidential election in Romania, defeating a hard-right candidate who is aligned with President Trump and has opposed military aid to Ukraine.

With more than 98 percent of ballots counted, preliminary official results gave 54 percent of the vote in the presidential runoff to Mr. Dan, 55, the mayor of Romania’s capital, Bucharest. His opponent, George Simion, a nationalist and fervent admirer of Mr. Trump who had been widely seen as the front-runner, drew only 46 percent.

As he slipped behind Mr. Dan in early counting, Mr. Simion told supporters that “we are the clear winners of these elections.” He called for national protests should the final count show him as the loser, railing against what he said was an attempt “to steal the victory of the Romanian people.”

Mr. Dan’s victory will likely calm fears in Europe’s political mainstream that Romania, which borders Ukraine and plays a vital role in defending NATO’s eastern flank against Russia, might join Hungary and Slovakia in opposing help for Ukraine and in cozying up to Moscow.

But it will likely inflame Romania’s nationalist camp and its supporters abroad, including Vice President JD Vance, and stoke accusations that the system is rigged. Last year, a Romanian court ordered a last-minute cancellation of a presidential election that an ultranationalist appeared well positioned to win.

In the final days of the campaign, as opinion polls showed the race tightening, Mr. Simion laid the groundwork for a Romanian version of Mr. Trump’s “stop the steal” efforts in 2020. He insisted that only electoral fraud could prevent him from winning.

On Sunday, shortly before voting ended, Mr. Simion claimed that “many deceased people” had appeared on electoral lists, echoing the claims of Mr. Trump after he lost the 2020 election in the United States. Mr. Simion provided no evidence to support accusations that his victorious rival had benefited from fraud.

A mathematics prodigy in his youth who earned a Ph.D. in France before becoming a professor in Bucharest, Mr. Dan campaigned as a moderate conservative committed to both the European Union, which Romania joined in 2007, and to NATO, which it has been a member of since 2004.

Though supported, at least tacitly, by much of Romania’s political establishment, Mr. Dan ran as an independent and presented himself as an outsider untainted by close association with Romania’s two main political parties. Those parties have cycled in and out of power since the 1989 overthrow of communist dictator Nicolae Ceausescu.

Mr. Dan began his public career campaigning against corruption and the destruction of old buildings by real estate developers tied to corrupt politicians. Both candidates were involved in rallies in the early 2000s to save Bucharest’s historic center.

But while Mr. Simion embraced nationalist politics, campaigning to “unite” Romania with the former Soviet republic of Moldova, which is largely Romanian-speaking, and bits of Ukraine inhabited by ethnic Romanians, Mr. Dan pursued a more moderate agenda.

Mr. Dan helped found the Save Romania Union, a liberal party, but split with it over the issue of same-sex marriage, which progressives in the party supported. When Romania, at the urging of right-wing activists, held a referendum in 2018 on changing the Constitution to prohibit same-sex unions, Mr. Dan urged his party to stay out of the issue. The referendum failed because of low turnout.

Leftists view him as a conservative and nationalists as a sellout to the European Union, but his victory on Sunday indicated that voters wanted a middle path between bitterly polarized political camps.

Campaigning this past week, he said voters had a choice “between a democratic, stable and respected Romania in Europe — and a dangerous path of isolation, populism and defiance of the rule of law.”

Andrada Lautaru contributed reporting from Bucharest.