Prosecutors in Romania have opened a criminal case against Calin Georgescu, an ultranationalist politician who in November won the first round of a presidential election only to have his victory annulled when the Constitutional Court ordered a redo of the vote.

The start of criminal proceedings against Mr. Georgescu, a fierce critic of NATO and the European Union, was announced by prosecutors on Wednesday. The move came just two weeks after Vice President JD Vance cited Romania in a speech at a Munich security conference as an example of what he said was Europe’s “retreat” from free speech and democracy.

Romania’s Constitutional Court canceled the election last year just two days before a December runoff vote that Mr. Georgescu had been seen as well positioned to win. It said it wanted “to ensure the correctness and legality of the electoral process.”

The court’s intervention came after Romania’s security service released declassified intelligence reports that pointed to possible Russian interference in the election campaign but provided no solid evidence of that.

Mr. Georgescu stunned Romania’s political establishment by winning a first round of voting on Nov. 24, aided by a flood of videos on TikTok and other social media. A former soil expert with the United Nations, Mr. Georgescu was largely unknown in Romania at the start of the election campaign last year.

The videos promoting Mr. Georgescu ahead of the November vote were not labeled as campaign material, a violation of Romania’s election law, and it has not been explained how his campaign managed to dominate social media so suddenly and thoroughly.