Washington ? House Democratic leaders vowed Thursday to pass legislation setting a deadline of Sept. 1, 2008, for the withdrawal of U.S. combat troops from Iraq, a challenge to President Bush’s war policy that drew a blunt veto threat in return.

“It would unnecessarily handcuff our generals on the ground, and it’s safe to say it’s a nonstarter for the president,” said White House spokesman Dan Bartlett.

Little more than two months after Democrats took control of the House and Senate, Speaker Nancy Pelosi of California said the bill would set “dates certain for the first time in the Congress for the redeployment of our troops out of Iraq.”

Officials said the deadline would be accelerated – possibly to the end of 2007 – if the government of Iraqi Prime Minister Nouri al-Maliki failed to meet commitments for taking over security operations, distributing oil revenue and opening his nation’s constitution to amendments.

Pelosi said Democrats would add their war-related provisions to the administration’s request for nearly $100 billion to pay for the fighting in Iraq and Afghanistan.

The plan is to bring the bill to a vote by the end of the month, making it the first major test of the Democrats’ power since they rode a wave of anti-war voter sentiment to midterm election victories last fall.

Across the Capitol, Senate Democrats readied a less sweeping challenge to the commander in chief.

Their version would set a target date of March 31, 2008, for the withdrawal of combat troops – but no deadline. The measure says U.S. forces could stay beyond that date only to protect U.S. personnel, train and equip Iraqi forces and carry out counterterrorism operations. “We can’t stay in Iraq forever,” said Senate Majority Leader Harry Reid, D-Nev.

Reid has considerably less leeway than Pelosi, since Senate rules give Republicans greater power than their counterparts have in the House.

Presidential politics also figure in his calculations. Illinois Sen. Barack Obama, a candidate for the White House, told reporters the measure includes some of the key provisions of a bill he introduced earlier this year setting a March 31, 2008, target for withdrawal. “It expresses the central insight that we can’t have our troops policing a civil war,” he said.

Of the 141,000 U.S. troops in Iraq, roughly 60,000 are combat forces and the rest are support troops.

Bartlett attacked the House measure in comments to reporters aboard Air Force One as Bush flew to South America. “Obviously, the administration would vehemently oppose and ultimately veto any legislation that looked like what was described today,” he said.

House Republican Leader John Boehner of Ohio led the GOP counterattack.

“General (David) Petraeus should be the one making the decisions on what happens on the ground in Iraq, not Nancy Pelosi or John Murtha,” Boehner said, referring to the Pennsylvania Democrat who has been heavily involved in crafting legislation designed to end U.S. participation in the war.