BUCHAREST—On a recent day, nationalist politician Călin Georgescu made his way through a crush of people to enter a district court in Romania’s faded belle epoque capital to argue that he should be free to run in coming presidential elections.

Calin Georgescu, the far-right frontrunner in Romania’s presidential vote, has been barred from running in May’s election, in an explosive decision that sparked violence on the streets of Bucharest and came despite pressure from Washington to “listen” to voters’ demands.

Controversial Romanian politician Diana Sosoaca, who was barred by a top court from entering last year’s annulled presidential election, has filed her candidacy in an unlikely bid to enter in the May rerun.

The country’s electoral commission ruled on Sunday that Calin Georgescu, an outspoken critic of Ukraine and NATO, could not compete in the do-over election.

Romania’s largest far-right party is scrambling to find a candidate after the independent front-runner it supported was barred from May’s presidential election.

It’s a close question, given the role of Russia and the tender age of the country’s democracy.

By Iulian Ernst in Bucharest The leader of the Alliance for the Union of Romanians (AUR), George Simion, will replace Calin Georgescu as the leading far-right candidate in the May presidential elections,