Paul E. Singer is the founding partner of one of the oldest hedge funds around. And while he has become a major donor to Republican and conservative causes in recent years, he has largely managed to stay out of the limelight, even avoiding having his picture appear in newspapers.

But this year Mr. Singer became one of the biggest supporters of Rudolph W. Giuliani’s presidential campaign, making his jet available to Mr. Giuliani, while Mr. Singer and workers at his companies have donated $200,000 to the campaign. And he became the largest individual backer of a California ballot initiative that many Democrats believe could sink their chances of winning the presidency.

Suddenly, the normally low-profile Mr. Singer, a New Yorker, found himself singled out by Democrats intent on beating back the California effort before it gained any steam.

Howard Dean, the chairman of the Democratic Party, questioned “Paul Singer’s involvement in this dirty trick aimed at stealing the White House.” A group of Democrats filed a complaint with the Federal Election Commission charging that Mr. Singer had been acting on behalf of Mr. Giuliani in his efforts to change the California law — which Mr. Singer and the campaign deny. And the Democratic National Committee drew attention to the part of Mr. Singer’s business that involves buying the debt of poor countries at a discount and then seeking repayment in full — prompting an article in The Times of London labeling his firm, Elliott Associates, a “vulture fund.”