WASHINGTON - With approval this week of a war resolution, Congress has handed President Bush sweeping authority to use force against Iraq and has essentially removed itself from the decision to go to war.

Now, Bush has a broad mandate - achieved through an orchestrated campaign by his administration - to take pre-emptive action without further approval from lawmakers.

Under the measure, which the Senate approved early yesterday and the House on Thursday - both decisively - Bush could act after he officially informed Congress that diplomacy had failed and that military action in Iraq was consistent with the war on terrorism.

The House approved the resolution 296-133, with nearly two-thirds of Democrats opposing it. The Senate passed it 77-23, with a majority of Democrats in support.

The lopsided votes led to an air of confidence at the White House that it could show the United Nations that America is unified on the need to disarm Saddam Hussein, by force if necessary, of his weapons of mass destruction.

White House officials now hope for a strong show of support from the United Nations. There, Bush faces opposition to his plan to confront Hussein swiftly, though there were encouraging signs for him yesterday, including a statement by Russia that it might support a tough new U.N. resolution.

The resolution that Congress approved invokes the 1973 War Powers Act, which requires the president to consult with Congress before engaging in war. The resolution also requires that Bush report back to Congress every 60 days.

But under the 1973 law, once Congress has authorized the use of force, the president can use the military as he sees fit.

"Congress has spoken," said Ari Fleischer, Bush's spokesman. The president will consult with Congress as required, he said, but "the final determination about whether force should or should not be used rests with the commander in chief."

Some critics likened the measure to the broadly worded 1964 Gulf of Tonkin resolution. That resolution authorized President Lyndon B. Johnson to use force in Vietnam, and it paved the way for America's disastrous involvement in Southeast Asia.

The resolution Congress passed this week allows Bush to use force "as he determines to be necessary and appropriate" to "defend the national security of the United States against the continuing threat posed by Iraq" and to enforce U.N. Security Council resolutions.

Within 48 hours of using the authority, Bush would have to certify to Congress that all peaceful means of disarming Hussein had failed, and that attacking Iraq would be consistent with international efforts to fight terrorism.

But the requirements for the president are a formality. Bush has authority to invade Iraq when he chooses. "Congress is largely out of the picture," said Michael J. Glennon, a professor at Tufts University's Fletcher School of Law and Diplomacy. "It is, in effect, the functional equivalent of a declaration of war. And declarations of war generally don't have specific conditions and requirements - they are blank checks."

Many of the resolution's opponents expressed concern about a loss of congressional prerogatives. The Constitution vests power to declare war in Congress and power to command the armed forces in the president. The resolution, critics argued, would skew that balance in favor of the executive branch.

"Pass this resolution," Rep. Tom H. Allen, a Maine Democrat, said before the vote, "and Congress' role in this matter is finished as of this week."

That is not technically true. On military matters as on many others, Congress' main power lies in its allocation of money. The House on Thursday passed a $355 billion defense spending bill; the Senate is expected to follow suit next week.

If Congress opposes a military mission, it can deny funding for it. But two-thirds of the House and the Senate would have to support the move - the margin required to override a presidential veto. Congress tried without success to cut off funding for the Vietnam War as public support for that war flagged.

"It is very difficult politically to go against the president once a conflict is under way," said Glennon, of the Fletcher School. "You have people arguing that you'll be sending military troops into harm's way without giving them the resources they need."

For Bush, Congress' approval of broad authorization for him to wage war was a victory that had not seemed inevitable a month ago. Days after Bush delivered a speech at the United Nations, Iraq announced that it would allow the return of U.N. weapons inspectors.

Iraq's offer drained some of the momentum from Bush's effort to persuade lawmakers and foreign leaders of the urgent need to confront Hussein. Some members of Congress suggested giving inspectors time to go back into Iraq before supporting military action.

But over the past month, Bush spoke out nearly every day about the Iraq threat. White House aides made an aggressive push, bringing wavering lawmakers in to listen to the president. They also dispatched Defense Secretary Donald H. Rumsfeld and Bush's national security advisor, Condoleezza Rice, to Capitol Hill to lobby.

Administration officials pressed their case that the Sept. 11 attacks showed that America is more vulnerable to devastating threats from abroad than many citizens had recognized.

Despite the sweeping authority given to Bush, many lawmakers in both parties say they want the president to work closely with Congress. A group of moderate House Republicans has discussed asking Bush to address a joint session of Congress to outline his plans on Iraq.

"This seems fast to the American public," said Rep. Wayne T. Gilchrest, a Republican from Maryland's Eastern Shore. "The president has got to come to a joint session of Congress after the resolution is passed and tell us what he's planning."

Some others want Bush to seek a second congressional vote before any military action.

The resolution "does not require it," noted Rep. Roscoe G. Bartlett, a Western Maryland Republican. "But I urge the president to come back to Congress for an additional vote before he engages our military in a unilateral, pre-emptive attack against Saddam Hussein."

The administration argued that decisive backing from Congress would help secure approval of a U.N. resolution intended to disarm Hussein and authorizing "all necessary means" if he refuses.

That plan has been opposed by Russia and France, which both wield veto power on the Security Council. They have preferred to give weapons inspectors another chance before threatening force.

Yesterday, U.S. diplomats sought a compromise on the language of a new U.N. resolution. They offered to drop the threat of "all necessary means" to force Iraq's compliance and instead to warn simply of "consequences." Bush would still be able to interpret "consequences" to mean force, a U.S. official told the Associated Press.

France rejected the offer, remaining committed to two U.N. resolutions: One would authorize new weapons inspections; a second could approve force after further debate. U.S. officials oppose that approach.

The Associated Press contributed to this article.