ALBANY, N.Y. – A saxophone-playing former Green Party member who advocates for an immediate withdrawal of American troops from Iraq says he will challenge incumbent Hillary Rodham Clinton for the 2006 Democratic nomination for Senate.

“This is now a declared candidacy,” Steven Greenfield said in a telephone interview from his New Paltz home.

“She’s in favor of the war and in favor of continuing the occupation,” Mr. Greenfield, a professional musician, said.

A senior adviser to Mrs. Clinton, Howard Wolfson, declined to comment on Mr. Greenfield’s effort.

An independent pollster, Lee Miringoff, noting that the former first lady has already come under fire from some anti-war activists, including “Peace Mom” Cindy Sheehan, said Mr. Greenfield may not present a real threat to Mrs. Clinton but added that he could win some votes given Mrs. Clinton’s status as a potential 2008 presidential contender.

“Some folks might want to send a message to a national audience,” Mr. Miringoff, head of Marist College’s Institute for Public Opinion, said.

“Clearly, there is a clamoring in New York to change Iraq policy,” Mr. Miringoff added. “In some ways, how Clinton reacts to this, should he get any kind of footing, might be a precursor of 2008, because if the war is still going it would be surprising if there wasn’t an immediate-withdrawal candidate for president. So she’s a little vulnerable to that as sentiment grows for a major change in policy.”

Mrs. Clinton voted to give President Bush the power to go to war, and while she has been critical of his conduct of the effort, she has stopped short of calling for a troop withdrawal. Mrs. Sheehan, the anti-war activist and lifelong Democrat whose soldier son Casey was killed in Iraq last year, said last month that voters should not support Mrs. Clinton un less she comes out against the war.

Mr. Greenfield, who has an undergraduate degree in economics from Columbia University, said the anti-war sentiment may give him a chance even if current polls show Mrs. Clinton’s job approval rating well above 50% and have her far ahead of potential Republican rivals. She also had almost $14 million in her campaign kitty as of the end of September. Mr. Greenfield said he has raised no money so far.

“The most important poll numbers to me are the ones that say in excess of 80% of registered Democrats want the war to stop now,” Mr. Greenfield, who switched his party enrollment to Democrat just last month so he could challenge Mrs. Clinton in a September 2006 primary, said.

An October poll from the Quinnipiac University Polling Institute found 84% of New York Democrats believe going to war was wrong. The Hamden, Conn.-based pollsters did not ask voters if they wanted American troops out of Iraq.

“Realistically, do I think I can beat her? Honestly looking at it right now, I’d have to say the answer to that question is likely to be no,” Mr. Greenfield said.

But the potential challenger said, “as the situation in Iraq deteriorates,” he expects his support to grow.

“If you ask me that question next May, I would not be at all surprised if the answer becomes: ‘Well, it looks like we’re in a horse race,'” he said.

In 2003, as a Green Party candidate, Mr. Greenfield lost a bid for a seat on the Ulster County Legislature. The year before, he sought to unseat Rep. Maurice Hinchey, a Democrat. Mr. Hinchey got more than 113,000 votes. Mr. Greenfield, a Bronx native who grew up in Queens, got fewer than 3,000.

New Paltz is known for liberal activism and its mayor, the Green Party’s Jason West, made national headlines last year by marrying gay couples. State courts have ruled the ceremonies were illegal under New York law.

Mr. Greenfield, 44, moved to New Paltz five years ago. The volunteer firefighter’s campaign Web site shows a “before” photo of him being arrested at last year’s Republican National Convention in New York City and an “after” shot of him with his arms in the air after his release. While he plans to attend the state Democratic Convention in late May or early June of next year seeking support for his candidacy, Mr. Greenfield concedes he will likely have to petition his way onto the state’s September primary ballot by collecting the signatures of 15,000 Democrats statewide.