WASHINGTON – The House voted Thursday to give U.S. troops guaranteed time at home between deployments to Iraq, the latest but assuredly not the last challenge to President Bush from Democrats determined to end an unpopular war.

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.

The 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.