Sen. Christopher J. Dodd has been a friend to the nation's largest financial firms for nearly three decades, and they have been his most generous donors. But now he finds himself in political trouble, and is trying to prove the relationship is over.

In recent months, Dodd, chairman of the Senate banking committee, has traded quiet advocacy of measures favored by financial companies for a vocal crusade to rein in those very firms, whose excesses are widely blamed for the economic crisis. He has stopped accepting donations from banks supported by federal bailout money. And he has spent $500,000 on campaign ads that bash credit card companies as "loan sharks" and paint financial lobbyists as crybabies who think Dodd is "a big meanie" because he won't take their...