A somber Congress opened its historic debate yesterday over whether to authorize President Bush to use military force to drive Iraq out of Kuwait, or to insist that he wait longer for economic sanctions to work.

"The grave decision for war is being made prematurely," said Senate Majority Leader George J. Mitchell (D-Maine), in countering the president's bid for authority to use force with proposals urging continued reliance on economic and diplomatic pressure.

"There has been no clear rationale, no convincing explanation for shifting American policy from one of sanctions to one of war," he said.

Republicans responded that the risks of delay outweigh the risks of war. "When patience and delay become foreign policy goals by themselves, they are no longer virtues . . . . Patience at any price is not a policy, it's a cop-out," said House Minority Leader Robert H. Michel (R-Ill.).

The White House was actively involved in lobbying both sides of the Capitol, with virtually every member of the Cabinet readied to present Bush's case that a vote backing his authority to use the military was now the last, best chance to persuade Iraqi President Saddam Hussein to withdraw.

White House officials said Bush "understands this is a vote of conscience" that differs from normal congressional votes where party loyalty, a variety of trades and other extraneous issues can affect the outcome. "We're not trading bridges or libraries here," one official said, but added, "This is very serious, and not some legislative ploy, when the president says this vote is vital to persuade Saddam that we are serious."

Officials were confident about the vote expected Saturday in the House, and several dozen members were called to a White House breakfast with Bush today.

But they were nervous about the Senate. Bush met with a dozen senators yesterday, most of them Democrats backing him or leaning that way, to determine the best way to get a majority in the 100-member body. "He's not looking for a home run. He's not looking for 70 votes; 51 will do," Sen. Richard G. Lugar (R-Ind.) said following the session.

Sen. Charles S. Robb (D-Va.), one of the Senate Democrats supporting Bush at the meeting, said the consensus was that both the administration and the Democratic leadership could each count on vote totals in the high 40s, with a half dozen or fewer still undecided.

As introduced in the House, the White House-backed proposal would sanction the use of force after the president informs Congress that the United States "has used all appropriate diplomatic and other peaceful means to obtain compliance by Iraq" with the United Nations resolution authorizing the use of "all necessary means" to obtain Iraqi withdrawal from Kuwait after Jan. 15.

Senate Minority Leader Robert J. Dole (R-Kan.) said after the White House meeting that he expected to offer an identical resolution, with a final decision due this morning at a meeting of Senate Republicans.

Later, Dole told the Senate that he wanted senators briefed by CIA officials on the question of whether sanctions would work without military force.

Democratic leadership resolutions introduced in the House and Senate oppose war at this time, and assert Congress's ultimate authority to declare it. But they do not rule out approval for combat if Bush seeks specific authority for it in the future. The resolutions set out procedures for speedy consideration of any future presidential request for authority to take the nation into war.

Many Democrats said, however, that they believe both houses are likely to go along with the president.

House Speaker Thomas S. Foley (D-Wash.) conceded that the president's position was strengthened by the failure of Secretary of State James A. Baker III's talks Wednesday with Iraqi Foreign Minister Tariq Aziz in Geneva. "I'm sure some members feel that the failure of the Baker mission makes it more likely they will vote for authorization" for military force, he said.

The long-awaited congressional debate opened first in the Senate shortly before noon, with no more than a half-dozen senators on the floor, although the public galleries were nearly full as Mitchell took the floor.

The question was not whether Iraq should be compelled to leave Kuwait, by force if necessary, but rather whether war should "be truly a last resort when all other means fail," said Mitchell.

In grave terms that have not often been heard in the sometimes-frivolous debates of recent years, Mitchell listed the risks: "An unknown number of casualties and deaths, billions of dollars spent, a greatly disrupted oil supply and oil price increases, a war possibly widened to Israel, Turkey or other allies, the possible long-term American occupation of Iraq, increased instability in the Persian Gulf region, long-lasting Arab enmity against the United States, a possible return to isolationism at home."

Contending that sanctions still have a chance to work, he said a "truly haunting question" for families of those who may die in war will be whether they died in vain. "For if we go to war now, no one will ever know if sanctions would have worked . . . ," he said.

Mitchell was challenged immediately by Sen. John W. Warner (R-Va.), ranking minority member of the Senate Armed Services Committee, who charged that the Democratic resolution would tell Iraqi President Saddam Hussein that the United States is divided, and would send a signal of weakness that ultimately could cost lives.

The best way to avoid war and any loss of life is for Congress to line up squarely behind the president to show that "we mean business and we're prepared to fight," added Sen. Arlen Specter (R-Pa.). If Congress fails to do this, "then our leadership in the coalition will fail completely . . . and the sanctions will collapse because of a lack of confidence in the United States" in the international coalition that this country organized to drive Iraq out of Kuwait, he said.

Gradually senators came to the floor to join the debate, occasionally arguing with great passion.

In his first Senate speech, newly sworn-in Sen. Paul Wellstone (D-Minn.) said he had been asked at town meetings, often by Vietnam war veterans, how many of his children were going to be fighting in the Persian Gulf.

"I would say I'm the son of a Jewish immigrant from the Soviet Union, and if I believed that Saddam Hussein was a Hitler . . . I could accept the loss of life of one of my children," aged 18, 21 and 25, he said. But that is not the case and he could not send his own children to battle, Wellstone said. "If I apply this standard to my children, then I have to apply that standard . . . to all of God's children," he added.

"President Bush appears to be on the verge of making a terrible mistake that will have tragic consequences for the whole world. Life is so precious, war is an option that you pursue after all other options have been tried. We have not given sanctions a chance," Wellstone concluded.

Sen. John C. Danforth (R-Mo.) spoke of the struggle within his own mind that first led him to conclude that war was the worst option but then convinced him that continuation of the present impasse is worse. Sanctions alone will not drive Iraq out of Kuwait, and "the key to peace is mainly a credible military threat," he said.

Sen. Daniel Patrick Moynihan (D-N.Y.) warned against what he called a "Dr. Strangelove" approach to war against Iraq, especially now that the Cold War appears to be over. "There seem to people who are saying, 'Oh my God, we missed World War III. Maybe we can have it here,' " Moynihan said.

Arguing that international sanctions are working, Sen. Paul S. Sarbanes (D-Md.), a member of the Foreign Relations Committee, said, "This is the time to show the courage to stay the course . . . the courage of patience."

In the House, the debate began shortly after sundown, but was foreshadowed by arguments before the Rules Committee as it set the parameters for the debate, most of which will occur today.

"This is one of the defining votes that each one of us will cast in our congressional careers," said Rep. Lee H. Hamilton (D-Ind.), chief sponsor of the Democratic leadership proposal.

"Unlike the Gulf of Tonkin resolution {used by the Johnson administration to justify expansion of the Vietnam War} . . . no one is going to be able to say, 'I was deceived; I didn't know what I was voting for,' " said Rep. Stephen J. Solarz (D-N.Y.), who is the lead sponsor of the bipartisan resolution backed by Bush.

In a series of news conferences before the debate started, lawmakers began outlining arguments that would unfold later on the House floor.

A defeat for the president, said Rep. Henry J. Hyde (R-Ill.), "would really be a setback for the allied side and a confirmation for Saddam Hussein that he could continue to play Congress off the White House."

House Majority Leader Richard A. Gephardt (D-Mo.) argued that "sanctions are powerful tools and they are achieving our objectives without the further loss of American life."

On the House floor, Rep. Sam Gibbons (D-Fla.), a veteran of the Normandy invasion in World War II, argued that now is not the time for another war. "Let there be no doubt about what is being done here: we are being asked to declare war," he said. "Oh yes, it doesn't have the ribbons on it . . . but it is just as strong a declaration of war as I've seen in my lifetime."

Michel, another World War II combat veteran, spoke from a different perspective: "Those of our generation know from bloody experience that unchecked aggression against a small nation is a prelude to international disaster . . . . Let no one in this chamber or anywhere else lecture me on the horrors of war. I've seen it at its worst."

Staff writers John E. Yang and Ann Devroy contributed to this report.