WASHINGTON (AP) – Breaking a parliamentary roadblock, the Senate on Wednesday began its first formal debate on the Iraq war since Democrats took control of Congress, taking up a Democratic resolution calling for President Bush to withdraw American combat troops by the end of next March. The White House swiftly issued a veto threat.

The 89-9 vote paved the way for consideration of the Democratic legislation, which would start troop withdrawals within four months and calls for – but does not require – the complete removal of combat troops by the end of March 2008. The vote came after many Republicans abandoned the tactic they had used twice earlier this year to prevent the Senate from considering legislation aimed at forcing an end to the war.

Despite the vote, most Republicans opposed the Democratic bill and it was expected to eventually fall short of the 60 votes it will need to pass. Even so, the debate would give Democrats a chance to put Republicans on record as opposing a timetable on the war at a time when most American voters oppose.

“This is the message the American people delivered to Congress on Nov. 7, 2006, and this is the message we must send to President Bush,” said Senator Reid, Democrat of Nevada and the Senate Majority Leader, referring to an election day in which Democrats captured both chambers.

The Senate breakthrough came after Republicans abandoned demands for assurances that a debate on the war include consideration of various GOP proposals, including a resolution vowing to protect funding for troops. Fearful such a measure would undercut the anti-war message Democrats wanted, Senate Democrats had refused.

But confident the latest Democratic proposal would fail, Republicans agreed to let debate begin. Republicans have argued that Congress should give the troop increase Mr. Bush ordered in January time to work. Mr. Bush says the increase – 21,500 combat troops plus thousands of additional support troops – is needed to help stabilize Iraq, where American forces are now commanded by General David Petraeus.

“It is a clear statement of retreat from the support that the Senate only recently gave to General Petraeus,” said Senator McConnell, Republican of Kentucky and the Sneate Minority Leader, contrasting the Democratic measures with the chamber’s recent approval of Petraeus’ nomination as commanding general of the Iraq war.

The White House said the resolution “infringes upon the constitutional authority of the president as commander in chief by imposing an artificial timeline to withdraw American troops from Iraq, regardless of the conditions on the ground or the consequences of defeat,” according an administration statement.

The Senate measure is weaker than legislation being considered by House Democrats that would demand troops leave before September 2008. However, several Senate Democrats have been reluctant to impose a strict deadline on the president.

In the House, Democratic leaders continued to try to rally members behind spending legislation aimed at ending the war. The House passed a nonbinding resolution in February stating opposition to Mr. Bush’s decision to send 21,500 more troops to Iraq.

The $124 billion measure would includes $95.5 billion for military operations in Iraq and Afghanistan. The money for the Defense Department is $4 billion more than the president requested – extra money intended to enhance operations in Afghanistan and pay for added training and equipment and improved medical care for American troops.