Bush threatened to veto the measure, which passed on a vote of 229-194. Six Republicans broke ranks to support it and three more voted "present" rather than take a firm position. Hawai'i Democrats Neil Abercrombie and Mazie Hirono voted in support of the measure.

House Democrats staged the vote as Defense Secretary Robert Gates became the latest administration official to acknowledge miscalculations about Iraq and a national public opinion poll said support for a troop withdrawal exceeds 60 percent.

En route home from the Middle East, Gates acknowledged the slow pace of political reconciliation among Iraqi leaders. "In some ways we probably all underestimated the depth of the mistrust and how difficult it would be for these guys to come together," Gates said.

The House measure would require that regular military units returning from the war receive at least as much time at home as they spent in Iraq. Reserve units would get a home stay three times as long as they spent in the war zone.

Under the Pentagon's current policy, active duty troops typically serve deployments of up to 15 months, with a year at home in between. National Guard and Reserve ground units generally can be called for as long as two years, to be followed by six years at home.

Since taking power in January, congressional Democrats have moved methodically, but so far unsuccessfully, to force Bush to change course in the war. Most prominently, the president vetoed legislation this spring that included a timeline for a troop withdrawal.

A Pew Foundation poll released Thursday showed support for a withdrawal is strong — more than half the country as a whole, including 85 percent of self-described Democrats and 64 percent of independents.

After devoting significant time and energy to the war earlier this year, Congress largely has focused on domestic legislation in a run-up to a monthlong vacation. Democratic leaders have said repeatedly they will renew their challenge to Bush in September, when Gen. David Petraeus, the top U.S. commander in Iraq, delivers a long-awaited report on the state of the conflict.

The legislation that House Speaker Nancy Pelosi, D-Calif., ordered brought to the full House during the day was largely symbolic, allowing Bush to disregard the required intervals between troop deployments in the interest of national security.

Still, it appeared designed to be politically painful for Republicans. They were forced to choose between supporting the president or leaving themselves open to charges they were uncaring for the troops and their families.

"This is all about the troops and their loved ones," said Rep. Ellen Tauscher, D-Calif.

Rep. Stephanie Tubbs Jones, D-Ohio, said most Americans who are killed "are on their second or third or fourth tours of duty. If all it takes to save their lives is to give them some rest, give them some rest."

Republicans said — and Democrats did not deny — that the measure would complicate the Pentagon's efforts to maintain current troop levels.

Rep. Howard (Buck) McKeon, R-Calif., said the legislation was "a backhanded attempt to force a U.S. withdrawal from Iraq." He noted the requirement for time at home did not apply to troops deployed to the war in Afghanistan. "If this were a sincere effort ... it would apply to all deployments," he said.

Rep. Duncan Hunter, R-Calif., said passage would threaten the safety of the troops rather than protect them because the measure would arbitrarily leave units at home that had specialized skills needed in the war.

In his remarks to reporters, Gates said the "depth of mistrust" among factions in Iraq is greater than U.S. officials anticipated.

Talking to reporters on board his plane as he returned from a four-day swing through the Middle East, Gates said he is more optimistic about improvements in security in Iraq than he is about getting legislation passed by the bitterly divided government.

Earlier this week, six Sunni Cabinet ministers quit the Baghdad government in protest.

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