President Bush formally notified Congress yesterday that he had ordered U.S. troops to the Persian Gulf, taking the first step in what could become another constitutional standoff between the White House and Congress over the controversial 1973 War Powers Resolution.

That law, prompted by the 1970 invasion of Cambodia and enacted over President Richard M. Nixon's veto, requires the president to notify Congress within 48 hours of dispatching troops where "imminent involvement in hostilities is clearly indicated" and calls for troops to be withdrawn after 90 days at most, unless Congress authorizes the operations.

The law has caused repeated struggles between Congress and the White House because of an inherent tension between two provisions of the Constitution. One gives Congress the sole power to declare war. The second declares the president commander-in-chief of the armed forces.

Nixon and all presidents since 1973 have said the War Powers Resolution is an unconstitutional infringement on presidential authority. For that reason, they have never officially triggered the 90-day clock set out in the law, even while complying with a provision that Congress be notified within 48 hours when troops are dispatched.

The last confrontation came over President Ronald Reagan's commitment of naval forces as escorts to Kuwaiti ships flying the U.S. flag for protection against Iranian forces in the Persian Gulf in 1987. The Senate voted down a motion to invoke the law on the operation.

Bush, who called for the law's repeal during his election campaign, said in his letter to the House and Senate leadership that he did "not believe involvement in hostilities is imminent; to the contrary, it is my belief that this deployment will facilitate a peaceful resolution of the crisis." Bush said the letter was "consistent" with the 1973 law, but he did not invoke the law since he argued that hostilities were not "imminent."

Senate aides said Congress may regard the letter as adequate notification required by law and conclude that Bush is in compliance, at least for the time being. But the wording indicates that, as a legal matter, Bush reserves the right to argue that he is not acting pursuant to the law and not triggering the 90-day clock.

With congressional support for Bush's actions in the gulf nearly unanimous so far, the War Powers Resolution has not become an issue on Capitol Hill. Still, Senate Foreign Relations Committee Chairman Claiborne Pell (D-R.I.) warned that in the event of hostilities, Bush must obtain formal congressional approval. "That is the law," Pell said.

Sen. Brock Adams (D-Wash.), one of the law's strongest supporters, said he hoped Bush would invoke the law because Congress would back him this time and that would "unify the country in what could be a potentially difficult situation."

The law is "the perfect tool" for rallying congressional support for what might be a "long-term commitment of U.S. forces on foreign soil." Adams said Bush "shouldn't be afraid to use it, and it would carry the country forward."

The Supreme Court in 1983 effectively struck down part of the law that gave Congress the authority to countermand the president by passing a joint resolution. Federal courts, citing procedural and other difficulties, have ducked congressional lawsuits aimed at getting the judiciary to start the 90-day clock.

As a result, Harvard Law School professor Laurence H. Tribe said, a "consensus has developed" that the law is unenforceable and "largely a dead letter." The president can dispatch troops to trouble spots, Tribe said, and Congress can do nothing to stop him short of impeachment or cutting off funds. Cutting off funds, he said, is unrealistic, because that would leave "troops without support."

Congress, even if it could muster the political will, does not have the constitutional power to pass a law requiring the president to make or not make specific troop movements, Tribe said.