ROME — Umberto Bossi, the colorful leader of a populist party best known for its anti-immigrant rhetoric, resigned on Thursday amid a widening scandal of illegal party financing.

Famous for his cigars, white tank-tops and salty language, Mr. Bossi once provided crucial support to the coalition of the former prime minister, Silvio Berlusconi. His resignation as head of the once-secessionist Northern League seemed to signal a turning point in Italian politics, coming months after Mr. Berlusconi stepped aside to make way for the technocratic government of Prime Minister Mario Monti.

After similar recent corruption scandals in a left-wing party, the news was a further sign of the political uncertainty in Italy today, which is marked by a growing divide between the political parties and the rest of society — under the shadow of an economic crisis whose depth and breadth Italians have only recently begun to grasp.

“It’s the end of a cycle. After the end of Berlusconi, now Bossi leaves, too,” said Roberto D’Alimonte, a political science professor at Luiss Guido Carli University in Rome. “It’s really the end of the cycle of the Second Republic,” he added, referring to the Berlusconi years. (The First Republic refers to the postwar political order, which ended in the early 1990s, followed by the rise of Mr. Berlusconi, a media magnate turned politician.)