Congressional leaders cleared the way yesterday for a full-blown debate on U.S. policy in the Persian Gulf before Jan. 15.

Senate Majority Leader George J. Mitchell (D-Maine) said the Senate would begin its gulf debate as early as next Thursday -- the day after Secretary of State James A. Baker III is scheduled to meet with Iraqi Foreign Minister Tariq Aziz -- and not later than Jan. 14, one day before the U.N.-imposed deadline for Iraqi forces to withdraw from Kuwait.

"There must be a debate and votes on this subject," said Mitchell, who, along with other Democratic congressional leaders, has insisted that President Bush cannot commit U.S. forces to offensive operations without the prior authorization of Congress.

Senate Armed Services Committee Chairman Sam Nunn (D-Ga.), who has urged the administration to give economic sanctions more time to work before resorting to war, said he agreed with Mitchell, but cautioned that debate must not be dragged out.

"If the president decides he's giving up on sanctions and diplomacy," Nunn said in an interview, "he's got an obligation to come to Congress and ask for authorization to use force. But Congress has an obligation to act expeditiously -- one day, or two or three, at the most. The worst case would be if the president made up his mind and then we had several weeks of debate. That would make our constitutional procedures look ridiculous in the eyes of the world and of the people at home."

On Thursday, congressional leaders said they would try to hold off a full debate until Baker's mission was formally scheduled and completed. They had spoken of waiting until Jan. 23. The accelerated schedule yesterday was prompted by word that a date had been set for a Baker-Aziz meeting.

Meanwhile, several senators skeptical of administration policy continued the debate that began Thursday on the president's authority to start combat operations against Iraq, pushing their arguments that only Congress has the constitutional authority to begin a war with Iraq.

"Now is the time and here is the place to debate this issue, not after the bullets start flying," said Sen. Tom Harkin (D-Iowa), a cosponsor of a resolution that would prohibit Bush from attacking Iraq without explicit congressional authorization.

Lawmakers in the House and the Senate have been vigorously pushing their leadership for an early debate and vote on the war power issue, arguing that to wait risks being overtaken by events.

"If we start this debate too late and the president takes action," said Sen. Brock Adams (D-Wash.), "the debate will be entirely different."

Similar pressures are building up on the House side, where Speaker Thomas S. Foley (D-Wash.) promised a large group of liberal lawmakers Thursday night that he would begin a House debate prior to any hostilities between the United States and Iraq.

Reps. Richard J. Durbin (D-Ill.) and Charles E. Bennett (D-Fla.), who have introduced a resolution in the House as a companion to that sought by Adams and Harkin in the Senate, announced yesterday they now have 51 supporters. A similar resolution was earlier adopted by the House Democratic caucus, 177 to 37.

But Republican supporters of the White House yesterday belittled the debate over the constitutional war-making power as "hand-wringing" that obscured the central issue of forcing Iraq out of Kuwait, which it occupied Aug. 2. "The issue isn't the Constitution," said Sen. Alfonse D'Amato (R-N.Y.), "it's the vital, bottom-line, live-or-die, long-term national interests. Let's stop the hand-wringing."

Senate Minority Leader Robert J. Dole (R-Kan.), noting that Iraq had accepted an invitation for its foreign minister to meet with Baker in Switzerland, said Congress has a "responsibility to give Secretary Baker a chance to deliver his message to Aziz, or to {Iraqi President} Saddam {Hussein}, or whoever, without having that message dis-torted or drowned out by the conflicting voices of a hundred senators."

In a related development, Comptroller General Charles A. Bowsher told the House Budget Committee that the cost of Operation Desert Shield would reach $30 billion this year absent any hostilities, an estimate that is consistent with Pentagon projections. Combat could multiply that figure many times, according to the General Accounting Office, the congressional auditing agency that Bowsher heads.

Staff writer David S. Broder contributed to this report.