WASHINGTON - With concern rising in Congress about a possible U.S. invasion of Iraq, President Bush will meet today with key members of Congress to discuss the threat posed by Saddam Hussein's regime.

The meeting will mark the first of several that Bush and his deputies plan to hold with top lawmakers to make their case to Congress - and, by extension, to the American public - that Hussein could soon threaten the world with nuclear weapons.

The White House session comes as congressional Republicans and Democrats, back in Washington after a monthlong break, are expressing deep skepticism about the need to launch a pre-emptive attack on Iraq and insisting that Congress be involved in any such decision.

"Most Democrats believe that the president has yet to make the case for taking action in Iraq," said Senate Majority Leader Tom Daschle, a South Dakota Democrat.

Bush is facing questions from fellow Republicans as well.

Senate Minority Leader Trent Lott of Mississippi said Congress supports a regime change in Iraq, calling Hussein "a major problem in the world today and a danger for the future." But when asked whether he is satisfied with how the White House has articulated its stance, Lott said, "I'd like to have a couple more days before I respond to that."

Other Republicans have gone further, openly challenging the need for a U.S. invasion. Among them is Sen. Chuck Hagel of Nebraska, a Vietnam veteran who sits on the Foreign Relations Committee and is a leading voice on defense and foreign policy.

House Majority Leader Dick Armey of Texas also has warned against an unprovoked U.S. attack. He told CNN on Sunday: "We are not an aggressor nation. I don't want us to ever be."

Though the president has not said so explicitly, administration officials have said that Bush is likely to seek formal congressional approval before launching any attack on Iraq.

"Technically, the president is not required to come to Congress," said Sen. John McCain, an Arizona Republican. "Politically, it would be foolish not to."

Secretary of State Colin L. Powell, who is known to be highly concerned about the consequences of a pre-emptive strike, declined to delve into details yesterday of the Bush administration's position.

"The president will articulate it," Powell said. "He will articulate it fully and in the near future."

Senators from both parties outlined a plethora of questions yesterday about the use of force that they want answered. Among them:

What intelligence does the Bush administration have that points to threats from Iraq? Where do U.S. allies stand? What might the costs of military action be? What regime would replace Hussein's, and what role would the United States play in a reconstruction of Iraq?

Bush is expected to begin laying out answers this morning at the White House meeting, which will include House and Senate leaders, as well as top members of foreign relations, defense and intelligence committees. A discussion of Iraq will continue on Capitol Hill, with Defense Secretary Donald H. Rumsfeld scheduled to provide senators with a classified briefing today.

This fall, the House International Relations Committee is to hold hearings, and the Senate Foreign Relations panel is set to convene a second round to follow hearings it held in July. The Senate Armed Services Committee may also hold hearings.

While no Bush administration officials testified before the Foreign Relations Committee in July, Rumsfeld suggested that the president might decide to lay out new evidence now of why Hussein poses such a threat and must be ousted.

As the administration has intensified its rhetoric on Iraq, some lawmakers said they have heard rising concern in their home states this summer about the potential costs and consequences of an offensive against Hussein.

"People are very much engaged in discussion about Iraq in every forum, and in most cases, it eclipses everything else," said Sen. Richard G. Lugar, an Indiana Republican and senior member of the Foreign Relations Committee. "They're deeply concerned about the basis by which we might go to war."

Indeed, a new ABC News poll found that public support for an attack has fallen to its lowest level since the war on terrorism began. The poll found that 56 percent favor military action against Iraq, down from 69 percent in early August. And 39 percent support a U.S. attack even if U.S. allies oppose it, down from 54 percent last month.

Sen. Dianne Feinstein, a California Democrat, said she has "many questions" about the notion of a U.S. invasion, especially given the lack of public support from America's allies.

"It's a step, I think, that is going to unleash a chain of events that is well beyond our current thinking," said Feinstein, mentioning a potential backlash in the Arab world against the United States and an undermining of the war on terror.

She has introduced a nonbinding resolution expressing House and Senate opposition to the use of force in Iraq without congressional authorization

Daschle declined to say whether the Senate would act on Feinstein's resolution, co-sponsored by Patrick J. Leahy, a Vermont Democrat, "until after we've had more briefings."

White House lawyers have told the president that "from a strictly legal point" he would have the authority to act, said Bush spokesman Ari Fleischer.

"But that's a very different cry from what the president may ultimately do if he decides to take military action," Fleischer said. "The legal issue is a different issue from the practical issue."

Administration officials sought to clear up some ambiguity from recent days, saying the White House fully supports the return of United Nations weapons inspectors to Iraq. But Rumsfeld said he remains skeptical that Iraq would allow the inspectors to return unconditionally.

"I haven't seen any inclination on their part to agree to anything except as a ploy from time to time," Rumsfeld said.

Powell brushed aside a fresh offer from Iraq to begin discussing the return of weapons inspectors. The offer was made by Tariq Aziz, Iraq's deputy prime minister. Aziz said Baghdad would be willing to begin such talks once sanctions on Iraq were lifted and Iraq was granted sovereignty over all its territory.

"It's nonsense," Powell said. "It's the con the Iraqi regime, and especially Mr. Tariq Aziz, have been pulling on the international community for years. It's time for the international community to speak back."

Sun staff writer Mark Matthews and the Associated Press contributed to this article.