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.