The Bush administration pushed Congress today for a broad vote to authorize the president to use force against Iraq.

But a new element was injected into the debate by a C.I.A. assessment that Saddam Hussein, while now stopping short of an attack, could become "much less constrained" if faced with an American-led force.

The judgment was contained in a letter signed by the deputy C.I.A. director, John McLaughlin, on behalf of George J. Tenet, the director of central intelligence. It was alluded to in a hearing of a Congressional panel investigating the Sept. 11 attacks and then released tonight, after the House opened its debate on Iraq.

The letter said "Baghdad for now appears to be drawing a line short of conducting terrorist attacks" with conventional or chemical or biological weapons against the United States.

"Should Saddam conclude that a U.S.-led attack could no longer be deterred, he probably would become much less constrained in adopting terrorist action," it continued. It noted that Mr. Hussein could use either conventional terrorism or a weapon of mass destruction as "his last chance to exact vengeance by taking a large number of victims with him."

The letter dated Oct. 7 also declassified an exchange from a closed Congressional hearing on Oct. 2 in which a senior intelligence official judged the likelihood of Mr. Hussein's initiating an attack in the foreseeable future as "low."

Mr. Tenet said tonight that "there is no inconsistency" between the C.I.A. views in the letter and those of the president. He emphasized the Iraqi leader's use of such weapons against his own citizens.

Senior administration officials insisted that the letter did not contradict President Bush's assertions on the imminent threat posed by Mr. Hussein. They pointed to another section of the letter that noted that the likelihood of Mr. Hussein's using weapons of mass destruction "for blackmail, deterrence, or otherwise, grows as his arsenal builds."

The letter also cited credible reporting that Al Qaeda leaders sought contacts in Iraq who could help them acquire weapons of mass destruction and that Iraq has provided members of the terrorist group with training in the areas of poisons, gases and bomb making.

One lawmaker on the intelligence committee, Senator Ron Wyden, Democrat of Oregon, cited the letter today as he registered his opposition to granting the president broad authority to use force unilaterally. "I'm not convinced regarding a clear and present threat," he said in Senate debate.

Even so, his was a minority sentiment a day after President Bush told the nation that Iraq "stands alone" as a threat, armed with weapons of mass destruction controlled by a "murderous tyrant."

Bipartisan approval of a resolution on force that President Bush negotiated with Congressional leaders is now considered all but certain. But administration officials worked to expand their support so that the president would be able to say that his backing was resounding when the final votes are taken.

Secretary of State Colin L. Powell told Republican Senators at a closed-door caucus that Congressional unity would help him press his case at the United Nations for a tough new resolution holding Iraq to account for its violations of a raft of past United Nations resolutions.

"What I'm interested in seeing is solid, overwhelming support as a signal of American determination," Mr. Powell, flanked by Democrats and Republicans, told reporters.

Despite the administration's push there were strongholds of dissent. A fervent minority of determined lawmakers in both House and Senate argued in debate that it was a mistake to give President Bush such broad authority for force.

Senator Robert C. Byrd, Democrat of West Virginia, also served notice that he would use all the procedural weapons at his disposal to slow debate — a move that could push off a final Senate vote until next week.

Others also made their opposition known. Senator James M. Jeffords, the Vermont independent whose change of parties gave the Democrats control of the Senate last year, said today that he could not open the door to a unilateral military incursion by the United States.

"I fear that this administration is, perhaps unwittingly, heading us into a miserable cycle of waging wars that isolate our nation internationally and stir up greater hatred of America," he said.

In the House, the sharp divide among Democrats was on plain view, even as most Republicans who spoke supported the president. And Democratic opponents of unilateral action predicted that more than 100 members of Congress would vote against the war resolution.

Speaker J. Dennis Hastert of Illinois opened 21 hours of formal House debate, saying that before the Sept. 11 attacks Americans had lived in "splendid isolation," but that the nation now realized it could be touched by those in countries that formerly seemed distant.

"Is there a connection between Iraq and Al Qaeda?" asked Mr. Hastert, who sought to bring the issue directly to lawmakers, saying the Capitol itself had been a Sept. 11 target. "The president thinks so and based on what I have seen, I think so also."

Democrats took pains to stress that any differences were matters of conscience. "There is no party position on an issue of this gravity," said Representative Martin Frost, Democrat of Texas and party caucus chairman, who supports the resolution.

Still, many House Democrats criticized the reach of the current resolution and the broad latitude it would give the president. "War with Iraq will not bring peace to the Middle East," said Representative John Lewis, Democrat of Georgia. "War is easy. But peace is hard. Peace is right, and it is just and it is true."

Representative David E. Bonior, a Michigan Democrat who was one of three lawmakers who traveled recently to Iraq, asked, "By going it alone, what signal do we issue by tossing aside diplomacy?"

Other Democrats clashed with Republicans over the extent of American cooperation with Saddam Hussein in the 1980's, saying previous administrations had provided the Iraqi leader with the foundation of his biological weapons program.

"Sure he has biological weapons," said Representative Louise M. Slaughter, Democrat of New York. "We gave them to him."