WASHINGTON — Defense Secretary Gates told a House panel yesterday that America should know in a few months if the Iraqi government is making progress toward peace and whether America “is going to have to look at other alternatives and consequences.”

In stark contrast to predecessor, Donald Rumsfeld, Mr. Gates also said there was no doubt the Army and Marine Corps needed to be larger if they are to deal with future wars and give troops enough rest between combat tours.

“We need the full range of military capabilities,” including ground forces to battle large armies and nimble special operations troops to scout out terrorist threats, Mr. Gates told the House Armed Services Committee.

“We don’t know what’s going to develop in places like Russia and China, in North Korea, in Iran and elsewhere,” he said.

Mr. Gates testified alongside the chairman of the Joint Chiefs of Staff, Marine Corps General Peter Pace, as the House was gearing up for its first full-fledged debate on the Iraq war since the November 7 elections. House Democrats plan next week to bring to the floor a measure that would flatly oppose President Bush’s decision to send 21,500 more troops to Iraq.

Mr. Gates had said Tuesday 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.

Democrats and several Republicans say they oppose the troop buildup and that it is time the Iraqi government stepped up to defend its own country.

Tired of waiting for an opportunity to try to stop the war in Iraq, some Democrats say they want to use legislation approving billions of dollars in war spending to insist that Mr. Bush not send more troops or bring troops home by a certain date.

“The longer we delay taking action, the greater the failure in Iraq and the larger the cost in American blood and treasure,” said Senator Dodd, a Democrat of Connecticut, who supports legislation that would cap the number of troops allowed in Iraq. Senator Obama, a Democrat of Illinois, said he would push his legislation ordering troops out of Iraq by March 2008.

“This is not a symbolic vote,” Mr. Obama said of his proposal, which is backed by two House Democrats. “This is what I think has the best chance of bringing our troops home.”

Senator Kerry, a Democrat of Massachusetts who drafted a similar proposal, said Congress’s actions will be watched carefully by voters headed to the polls in 2008.

“If the Congress is going to procrastinate, if they refuse to debate the most important issue of our time … then that is going to be clear issue in the context of ’08,” Mr. Kerry said.

Democratic leaders in both the House and Senate have pledged to their rank-and-file that a vote on a nonbinding resolution opposing the troop buildup would merely be the first attempt to pressure Mr. Bush to shift course in the war. Other legislation will be binding, they said.

The majority leader, Rep. Steny Hoyer, a Democrat of Maryland, told reporters he hoped the measure in the House would attract Republican support.