WASHINGTON — House Democratic leaders charted their course as Defense Secretary Gates told a Senate committee that American forces might be able to start leaving Iraq before the end of the year — if daunting conditions including subdued violence and political reconciliation are met.

Mr. Gates told lawmakers that the buildup in troops is “not the last chance” to succeed in Iraq and that “I would be irresponsible if I weren’t thinking about what the alternatives might be.”

Appearing before the Senate Armed Services Committee, he said, “We at this point are planning for success.”

Mr. Gates and other administration officials have been sharply critical of efforts to pass legislation expressing disapproval of the increase in troops, even if the form is nonbinding.

That has had no noticeable impact on Democratic critics of the war, though. Some of them argue that President Bush’s policies have led to a situation in which American troops are thrust into the middle of a civil war in Iraq.

The Democratic-controlled House will vote next week on a non-binding measure opposing Mr. Bush’s decision to dispatch additional troops to Iraq, officials said yesterday, hastening a postelection clash between Congress and commander in chief.

The precise nature of the measure remains to be determined, the officials said, although a spokeswoman for Rep. Nancy Pelosi, Brendan Daly, said it would be “our opposition to the surge” in troops.

The Pentagon is in the midst of implementing Mr. Bush’s order to raise troop levels by 21,500, part of a plan to help quell sectarian violence in Baghdad.

Mr. Bush’s revised strategy has sparked strong opposition among Democrats, and officials said Ms. Pelosi, who is the House speaker, and Rep. Steny Hoyer, who is the majority leader and a Democrat of Maryland, had both pledged to their rank-and-file that next week’s vote would merely be the first attempt to pressure the president to shift course in the war. Other legislation will be binding, they said.

Under House rules, Democratic leaders have the authority to advance a measure to the floor for three days of debate and a vote.

That stands in contrast to the Senate, where Republicans have so far blocked an attempt by Democrats to hold a full-fledged debate on a war that has claimed the lives of more than 3,000 American troops.

Even as the Democratic leaders mapped plans to take symbolic votes against Mr. Bush’s policy, two bills were unveiled during the day to force the president to move toward a troop withdrawal.

“The only people who believe there is a workable military solution for the conflict in Iraq is the Bush administration,” Senator Kerry, a Democrat of Massachusetts, said, announcing legislation that would require Mr. Bush to “complete the redeployment” of American troops within a year.

Senator Obama of Illinois, a Democratic presidential hopeful, joined two other lawmakers in proposing a measure to block Mr. Bush from implementing his planned troop increase, and to begin a withdrawal by May 1.