Senator Clinton faces a challenge from the left in her run for re-election: A former labor leader is mounting an anti-war bid for the Democratic nomination.

Jonathan Tasini, who headed the National Writers Union for 13 years, will launch his campaign today with a rally at the W Hotel in Union Square. He has touted the backing of noted liberal Democrats, including the anti-war activist Cindy Sheehan.

Mr. Tasini, 49, said in an interview yesterday he plans to make the Iraq war a dominant issue in a grassroots campaign. “The incumbent is not in step with what the majority of New Yorkers think, particularly on the war,” he said. “I think there’s a great base of people who are looking for something different.” He said he would also focus on pushing universal health care and opposing the free trade pacts Mrs. Clinton has supported.

The possible primary challenge comes as Mrs. Clinton has sought to reaffirm her position on the war amid growing calls from some Democrats for an immediate withdrawal of American troops from Iraq. Last week, in a 1,600-word letter to supporters, the senator defended her vote authorizing military action in 2002 but criticized President Bush for “mismanagement” of the war.

Mr. Tasini pledged support for Rep. John Murtha, the Democrat of Pennsylvania who last month called for a troop withdrawal after initially backing the war.

“The opposition to the war is across the political spectrum,” Mr.Tasini said.

Aides to Mrs. Clinton refused to respond to Mr. Tasini’s remarks or comment on his candidacy. “Senator Clinton is staying focused on New York,” an adviser, Howard Wolfson, said. Mrs. Clinton appeared at a job fair in Syracuse yesterday.

In announcing his bid, Mr. Tasini cited the support of Mrs. Sheehan, the “Peace Mom” who has for months led protests of the war and been a harsh critic of Mrs. Clinton’s position on Iraq. She said in a statement that she was “pleased” that Mr. Tasini had stepped forward as a candidate and advised New Yorkers to “take a hard look at Mrs. Clinton’s dismal record” on the war. Mrs. Sheehan, however, will not appear with Mr. Tasini today and said she could not officially endorse his candidacy, saying such a move could threaten the tax-exempt status of her nonprofit organization.

Mr. Tasini said he has been considering a run for the last 10 or 11 months. He aims to raise $3 million to $5 million for the race. “Our goal is not to try to match my opponent dollar for dollar,” he said. “We are dealing with a large political machine.”

Mrs. Clinton has amassed a $14 million war chest for her campaign. Her Republican opponent, Jeanine Pirro, is struggling to raise money, and some state party officials have suggested she drop her Senate bid and run for attorney general.

A longtime political consultant, Hank Sheinkopf, said Mr. Tasini’s entry into the race as an anti-war candidate could be significant. “The question becomes how much fire power he can bring to the anti-war argument,” he said, adding that if Mr. Tasini succeeded in drawing Mrs. Clinton to the left, it could harm her chances in the 2008 presidential race.

Still, Mr. Sheinkopf called Mr. Tasini’s fund-raising goals unrealistic. “He’s not Eugene McCarthy, and this is not the Vietnam War,” he said.

Mr. Tasini was born in Texas but has spent much of his life in New York. He is best known as the lead plaintiff in a 2001 Supreme Court ruling that found the New York Times and other publishers liable for copyright violations against freelance writers who charged the paper with posting their articles on Internet archives without their permission. The decision forced the Times to remove thousands of articles from Lexis-Nexis.