W By Janet Hook

ASHINGTON – The House on Tuesday joined the Senate in the sobering debate over authorizing the use of military force against Iraq, but a marked sense of uncertainty hangs over both chambers about whether the United States is, in fact, on the brink of war.

President Bush has insisted he has not decided whether he would use military force even if Congress authorizes it, and his top national security aides swarmed Capitol Hill to tell lawmakers that the administration will go to war only if diplomatic efforts aimed at Iraq fail.

“War is the last resort,” Secretary of State Colin L. Powell said as he headed into meetings with lawmakers at the Capitol. “I want to reassure the American people that neither the president nor Congress is leaping into something without thinking.”

In a letter made public Tuesday, the CIA said the probability of Iraqi President Saddam Hussein initiating an unprovoked attack in the near future, while he is still building an arsenal, is low. But if attacked, he would likely respond with biological or chemical weapons. The letter was signed on behalf of CIA Director George Tenet and sent to the Senate Intelligence Committee as part of a document declassification.

Tenet on Tuesday denied the letter contradicted the imminent nature of the Iraqi threat as described by President Bush in his nationally televised speech Monday night.

“Although we think the chances of Saddam initiating a WMD (weapons of mass destruction) attack at this moment are low – in part because it would constitute an admission that he possesses WMD – there is no question that the likelihood of Saddam using WMD against the United States or our allies in the region for blackmail, deterrence or otherwise grows as his arsenal continues to build.”

As debate opened in the House and continued in the Senate on the authority Bush seeks, his allies argued that the best way to avoid the need for a U.S.-led military strike is to deliver a strong bipartisan vote in support of the administration’s policy. They said that would strengthen Powell’s hand in building an international coalition to stand up to Saddam and force disarmament of his suspected weapons of mass destruction.

“It is through a strong show of support for this joint resolution that war can best be avoided,” Rep. Tom Lantos, D-Calif., said. “Against such an implacable foe as Saddam Hussein, peace can only be achieved through strength – the strength of conviction as much as the strength of arms. It is only when the Iraqi dictator is certain of our resolve and our ability that peace becomes possible.”

But many hawks in Congress and the administration say it is it plain that, based on Saddam’s record of noncompliance with U.N. mandates, the use of force against his regime is almost inevitable.

“Saddam Hussein’s track record is he’s never complied,” said Sen. John McCain, R-Ariz. “Everyone understands that if Saddam Hussein doesn’t comply, this means that the president can and probably will take military action.”

And some anti-war lawmakers argue that, despite the administration’s insistence that war is a last resort, the saber-rattling of recent months puts the United States irretrievably on the road to war.

“Preparing for war ensures it will truly happen,” said Rep. David Bonior, D-Mich., one of three anti-war Democrats who traveled recently to Baghdad, Iraq.

Lawmakers on all sides predict Bush will win the power he seeks to deal with Iraq when Congress takes its final vote on the issue. That vote is expected Thursday in the House, and perhaps by week’s end in the Senate.

Still, top administration officials have thrown themselves into lobbying wavering members to pump up Bush’s margin of victory.

Powell lunched with Senate Republicans on Tuesday then met with uncommitted House Democrats. National Security Advisor Condoleezza Rice briefed senators of both parties. A delegation of House fence sitters was brought to the Pentagon for a breakfast briefing by Secretary of Defense Donald Rumsfeld.

Rep. Jack Quinn, R-N.Y., one of those invited to the Pentagon, said the briefing – and Bush’s speech on Iraq Monday night – helped “move me further” toward supporting the president’s position.

The uncertainty about whether Bush will take military action against Iraq underscores the unusual ambiguity of the resolution before Congress. Bush is asking for authority to wage a war he has not yet decided to mount, so Congress is voting simply to allow Bush to decide later whether to attack.

Some of the resolution’s foes sought to focus attention on that point.

“The president is asking Congress to delegate its constitutional power to declare war before he has decided we need to go to war,” Sen. Edward M. Kennedy, D-Mass., said Tuesday. “But he has not adequately explained what this war will look like.”

The uncertainty also distinguishes the current debate from the one in 1991 on authorizing an attack on Iraq after it invaded Kuwait. The question then was not whether but when the United States would attack, and the military action began within days of Congress’ vote.

The House debate began with a rare floor speech by Speaker J. Dennis Hastert., R-Ill., who argued that inaction is not an option in the face of the perceived threat posed by Iraq.

“The only thing necessary for the triumph of evil is for good men to do nothing,” Hastert said, quoting Edmund Burke. “We must not let evil triumph.”

The House is expected to continue its debate Wednesday and vote Thursday on three alternatives:

ï Bush’s proposal, which would grant him broad authority to use any means he determines necessary and appropriate – including military force – to respond to any security threat posed by Iraq. It requires Bush to certify to Congress, within 48 hours of launching any attack, that diplomatic avenues had proved fruitless and that an attack was “consistent” with ongoing efforts to fight terrorism.

ï An alternative by Rep. John Spratt, D-S.C., that authorizes military force only in conjunction to passage of a new U.N. Security Council resolution. Bush would have to come back to Congress if the United Nations fails to act. A comparable measure by Sen. Carl Levin, D-Mich., will be voted on in the Senate.

A measure by Rep. Barbara Lee, D-Calif., that would urge the United States to work for Iraqi disarmament through peaceful means using U.N. diplomacy.