In his 30 years in the state Senate, Vincent J. Fumo has faced primary opponents only twice before, and he beat each of them easily.

This year, he faces three challengers and is the only incumbent senator in the city who has been forced to fight for the Democratic Party nomination.

Behind Fumo’s toughest-ever challenge is a belief that the powerful Democrat is especially vulnerable this year. And with some reason.

Fumo, 64, faces trial in September – just weeks before the general election – on a 139-count federal indictment charging him with corruption and fraud. Prosecutors allege that Fumo used his state staff and money from a neighborhood charity to support an opulent lifestyle and then orchestrated a cover-up after the FBI started asking questions.