A longtime labor advocate launched his challenge to Senator Clinton’s reelection while another anti-war Senate hopeful yesterday suggested the two join forces against Mrs. Clinton in the 10-month lead-up to the Democratic primary.

A former head of the National Writers Union, Jonathan Tasini, announced his bid in a 20-minute speech to supporters and members of the press at the W Hotel in Union Square. The address largely focused on his opposition to the Iraq war: He said Mrs. Clinton and other Democrats who voted for it “abdicated their responsibility to the American people and to the values of the Democratic Party.”

Mr. Tasini added: “People who supported the decision to go to war must be held accountable – and that includes my opponent.” He later criticized Mrs. Clinton for her opposition to an immediate withdrawal of American troops. Her position, he said, “closely resembles the Bush administration’s position.”

Mr. Tasini, 49, is the more prominent of two anti-war candidates looking to challenge Mrs. Clinton from the left in 2006. An activist and volunteer firefighter from New Paltz, Steven Greenfield, announced his bid in October.

In an interview yesterday, Mr. Greenfield, 44, said he reached out to the Tasini camp and proposed that the two work “synergistically” against Mrs. Clinton. He said it was obvious that neither antiwar candidate could match the senator in fund raising or political clout. Mr. Tasini “rejected this outright,” Mr. Greenfield said. Mr. Tasini said after his announcement yesterday that he was aware of Mr. Greenfield’s candidacy, but sought to position himself as a more serious challenger to Mrs. Clinton.

The incumbent senator, meanwhile, was in Saratoga Springs for a fund-raiser yesterday, where she was trailed by anti-war protesters, the Associated Press reported. When asked about her two challengers, she said she had “no argument with anyone who wants to run for any office,” according to the AP.