WASHINGTON — Senator Snowe, a Republican of Maine, said the political “tide has turned” on the Iraq war. She announced yesterday that she is backing forcing President Bush to withdraw American troops and predicted that more Republicans will abandon his war policy.

Ms. Snowe said she is likely to support at least one of several Democratic amendments to a defense policy measure that will demand a reduction in American forces. “We have to set it in motion,” she said of a withdrawal.

Ms. Snowe’s decision, five days after Senator Domenici, a Republican of New Mexico, called for a new American military policy in Iraq, reflects the wavering support for Mr. Bush’s war policy within the president’s own party in the Senate.

Until now, Ms. Snowe has indicated she would side with a Bush administration request that lawmakers wait until September when Army General David Petraeus, the lead U.S.commander in Iraq, is to give a report to Congress on the war’s progress.

Ms. Snowe said she is considering lending her support to an amendment, still being drafted by the Senate Armed Services Committee chairman, Senator Levin, a Democrat of Michigan, which would require troop withdrawals to begin within 120 days.

Ms. Snowe said she changed her stance after seeing few signs of progress in Iraq in recent months, and after learning of the death of another American soldier from her home state.

Senator Lugar of Indiana, the top Republican on the Foreign Relations Committee, and Senator Voinovich of Ohio have also recently called for a change in direction in Iraq in recent weeks. Six Senate Republicans are backing legislation introduced by Senator Salazar, a Democrat of Colorado, and Senator Alexander, a Republican of Tennessee, that implements the 79 recommendations of the Iraq Study Group.

While it doesn’t set a deadline for withdrawal, it aims to create conditions that could lead to a redeployment of American troops as early as the first quarter of next year.