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House Democrats on Thursday drew a veto threat from the White House after unveiling a plan that sets specific timetables to remove U.S. combat troops from Iraq and bring them home before fall 2008.

In a direct challenge to President Bush's strategy for Iraq, the plan would have troops redeploying as early as July of this year but no later than March 2008, and full combat withdrawal by August 2008.

At the same time Senate Democrats were preparing their own bill with binding legislation that would require a withdrawal from Iraq to begin no less than 120 days after the legislation is enacted with the goal of redeployment by March 31, 2008."

The Senate legislation, built on the recommendations of the Iraq Study Group, lists three exceptions. The slim Democratic majority in the Senate reduces its chances of success, and met an early roadblock Thursday afternoon when the Senate's top Republican used a procedural maneuver to keep the bill at bay for the time being.

The House legislation, to be attached to the $100 billion emergency supplemental war spending bill that Bush has requested to pay for ongoing military operations in Iraq and Afghanistan, is the first of its kind to establish a specific timetable for the end of U.S. combat in the four-year-old war.

The House Democrats' plan would pull out troops sooner — within four to six months — if Iraqi Prime Minister Nouri al-Maliki's government does not meet benchmarks established by President Bush. If Bush certifies the Iraqis have met the benchmarks, the operation can continue until the later date.

"We're talking about supporting the troops with the funding they need, honoring our promises to our veterans, holding the Defense Department to the standards they have about readiness ... holding the Iraqi government accountable for the benchmarks established by President Bush," House Speaker Nancy Pelosi said.

"If those benchmarks are not met ... (we will be) calling for redeployment of U.S. troops out of Iraq so that we can focus more fully on the real War on Terror, which is in Afghanistan," she said.

The plan would be binding, unlike a nonbinding resolution passed in February that rebuked Bush's war plans but had no legislative effect. The bill could be taken up in committee as soon as next week with hopes of sending it to the House floor the following week.

A senior administration official traveling ahead of the president to Brazil said Bush would veto any legislation that sets a timeline for withdrawal. The official said that such legislation would serve only to handcuff the commanders on the ground in Baghdad at a time when the generals in Iraq are seeing the Iraqi troops taking up more responsibility in the fight.

National Security Council spokesman Gordon Jondroe told radio reporters in Sao Paolo, Brazil, that the bill "goes absolutely counter" to the predictions of dire consequences from early withdrawal predicted by the most recent Nation Intelligence Estimate

Jondroe pointed to what Gen. David Petraeus said earlier Thursday as evidence that the Democrats' plan will not work, and he accused Democrats of "using political calculations" to craft a plan. Petraeus, the top U.S. commander in Iraq, pointed to early successes in Baghdad since a recent joint U.S.-Iraqi security crackdown got underway, but said a military solution alone will not settle Iraq, and a political solution is also important.

Shortly after the Democrats' announcement, Minority Leader John Boehner, the top House Republican voiced his dismay over the plan, said the proposal amounts to "establishing and telegraphing to our enemy a timetable" and would allow terrorists to respond to and wait out U.S. troop withdrawals.

"Gen. Petraeus should be the one making the decisions on what happens on the ground in Iraq, not Nancy Pelosi or John Murtha," Boehner added. Rep. Murtha has been a staunch critic of the war and has been calling for troop withdrawal since late 2005.

"House Republicans will not support the slow-bleed strategy they endorsed today," said Minority Whip Roy Blunt, R-Mo. "Command and control decisions on the ground in Iraq cannot be made efficiently or effectively by 535 commanders in chief sitting 6,000 miles away on Capitol Hill."

Democrats, still buoyed by their congressional takeover in the fall midterm election, have made several forays this year into checking the president's war plans. The president on Jan. 10 outlined his plan to send 21,500 combat troops into Iraq, mostly to Baghdad, to bring security to the country and allow the Iraq government to stabilize enough to handle security on its own.

In addition to the House rebuke in February, the Senate attempted its own nonbinding resolution, but failed on a rare Saturday work session. Murtha, D-Pa., presented his own solution, which would have limited troops that could be sent to Iraq by setting stricter training, equipment and rest requirements, but it did not gain traction.

With Senate Democrats again considering its own plan for withdrawal, it still must surpass the more difficult 60-vote, filibuster-proof hurdle that prevented nonbinding action in the chamber earlier this year.

"Troops should not be policing a civil war. The current conflict in Iraq requires a political solution," Majority Leader Harry Reid told reporters Thursday afternoon. "We further believe that Iraq must take responsibility for its own future and our troops should begin to come home."

Reid said the bill would accomplish those goals with by beginning a phased redeployment no more than 120 days after the resolution's enactment. Unlike the House bill, the Senate bill would not target the appropriations process, but nevertheless offers binding language.

The bill calls for redeployment by the end of March 2008, but says a limited number of combat forces would remain for the purposes of "(1) Protecting United States and coalition personnel and infrastructure, (2) Training and equipping Iraqi forces and (3) Conducting targeted counter-terrorism operations."

The bill calls for mandatory 90-day reporting requirements for the president — to start two months after the bill's passage. The proposal says the president "shall submit to Congress a report on the progress made in transitioning the mission ... and implementing the phased redeployment of the United States forces from Iraq as required under this section."

Moderate Democrats, like Sen. Ben Nelson of Nebraska, have said they would not accept anything that "sets a timeline for withdrawal," and about four or five other Democrats likely would also have trouble with this bill.

Reid, however, told reporters he will offering an olive branch to his GOP counterparts, by making a motion next week that will allow debate on three Republican amendments. Those amendments -- from Sens. John Warner, John McCain and Judd Gregg -- were a source of friction because Democrats had blocked earlier debate on them.

Asked if there would be enough votes to break a filibuster, Reid said: "You'll have to ask the Republicans. We have our votes."

But when Reid tried to bring the measure to the Senate floor Thursday afternoon, Minority Leader Mitch McConnell of Kentucky exercised a Senate rule that allows any senator to prevent debate on a measure.

McConnell said that the latest measure was the 17th plan that had been proposed since the opening of the 110th Congress.

"There was the Biden resolution. Then there was the Levin resolution. Then there was the Reid-Pelosi resolution. The Murtha plan. The Biden-Levin resolution. The Conrad funding cut. Then there was a waiver plan, a timeline plan, a Feingold resolution an Obama resolution," and plans from Sens. Hillary Clinton, Christopher Dodd, Edward Kennedy, Dianne Feinstein, Robert Byrd and John Kerry, McConnell said.

McConnell said he would have to consult with his GOP colleagues to see if the plan is something they could work with. Reid said the objection would delay the debate he planned to start Monday, but pledged to have an Iraq debate before the Easter recess and held out the threat of more weekend votes.

Back in the House, the divisions among Democrats over war policy is never far from the surface. Even as some prepared to unveil Thursday's plan, others made sure media were aware they were not yet on board with the leadership.

"This plan would require us to believe whatever the president would tell us about progress that is being made. This is the same president that led us into a war with false information, no weapons of mass destruction, said we would be welcomed with open arms, said the mission had been accomplished," said Rep. Maxine Waters, one of several members of the Congressional Progressive Caucus that is pushing to allow funding only to complete a withdrawal.

"We want a simple plan. We want a plan that will fund the safe and secure exit of our troops from Iraq in a reasonable amount of time and that's not asking too much," Waters said.

But Rep. David Obey, chairman of the House Appropriations Committee, said the proposal would bring an "orderly and responsible close" to American participation in what he called an Iraqi "civil war." More than 3,100 U.S. troops have died in the war since its 2003 start.

Instead of the more restrictive proposals Murtha initially set out, the new legislation will only call for the Pentagon to adhere to its own standards of combat readiness and time between combat tours.

The bill will also include waivers for Bush that would allow the president to continue with the plans he laid out in January. Democrats are planning to include some concessions as well as some other additions not strictly related to fighting in Iraq and Afghanistan.

Democrats are said to be increasing the amount of money to go toward fighting in Afghanistan, beyond what the president has requested.

The bill will exceed Bush's request for veterans' health care and medical programs for active duty troops at facilities such as the scandal-scarred Walter Reed Army Medical Center in Washington, D.C.

Democrats also are including funds for a health care program for low-income children. The program is popular among governors of both political parties, but the Bush administration has not signaled its acquiescence to the additional money.

The Democrats worked on their legislation as Defense Secretary Robert Gates announced approval of an extra 2,200 military police to help deal with an anticipated increase in detainees during the new Baghdad security crackdown.

And a senior defense official told The Associated Press Thursday that Lt. Gen. Raymond Odierno, the day-to-day commander in Iraq, has recommended that the higher troop level be maintained until February 2008 to support a sustained effort to win over the Iraqi populace. The official spoke on condition of anonymity because Odierno's request has not been approved.

FOX News' Mike Majchrowitz, Molly Hooper and Trish Turner and The Associated Press contributed to this report.