Senate Democratic leaders, brushing aside objections from President Reagan, introduced legislation yesterday to require congressional approval for indefinite continuation of the administration's controversial tanker-escort operation in the Persian Gulf.

It would terminate U.S. escorts of reflagged Kuwaiti tankers through the gulf within 90 days of enactment unless Congress approved legislation in the meantime to continue the operation.

The proposal, which is expected to be considered by the Senate today, would impose many requirements of the 1973 War Powers Resolution on the escort operation without formally invoking the law to limit U.S. military activities in the Persian Gulf.

"Hostilities have now occurred," fulfilling conditions laid out by the War Powers Resolution for congressional involvement, said Senate Majority Leader Robert C. Byrd (D-W.Va.) in introducing the proposal on behalf of a group of Democrats who included Armed Services Committee Chairman Sam Nunn (D-Ga.) as well as more liberal Democrats who previously advocated that the law be formally invoked.

"Instead of the blunt instrument of the war powers act, this amendment will act as a scalpel. It will place emphasis on the convoy and escort of reflagged tankers, not on the overall U.S. commitment," he added.

In shelving an earlier, milder version, the Democrats lost support of key Republicans for what both sides had originally hoped would be a bipartisan compromise. Sen. John W. Warner (R-Va.), ranking Republican on the Senate Armed Services Committee, denounced the proposal as "War Powers II" and said the Senate may face "prolonged" debate on it. Warner also said the White House "unequivocally" opposed it.

But the leadership won support of advocates of war-powers restraints such as Sen. Dale Bumpers (D-Ark.), who said he would "rather fall on my sword and go down in flames than do nothing," even if as expected the legislation produces a presidential veto.

The agreement occurred despite a letter from Reagan to Senate leaders yesterday that appeared aimed at heading off the action. It followed his warning Wednesday that it would be "grave mistake" for Congress to try to limit the duration of the escort operation.

In his letter, Reagan explained and defended the U.S. Navy attack Monday night on an Iranian ship that was laying mines in the gulf, saying he regarded "this incident as closed." He also asserted the president's constitutional authority to act unilaterally under the circumstances. But he did not acknowledge that the war-powers law applies to U.S. involvement in the Persian Gulf hostilities or meet other conditions raised by critics of his gulf policy. The effect of the letter on the drafting of the legislation was "minimal to none," according to a Democratic aide.

The Senate voted 50 to 41 last Friday against invoking the War Powers Resolution in the gulf, with Byrd, Nunn and 13 other Democrats joining most Republicans in voting against the move.

But pressure for action mounted in the wake of Monday's hostilities, resulting in another move by Sens. Lowell P. Weicker Jr. (R-Conn.) and Mark O. Hatfield (R-Ore.) to invoke the law and prompting Byrd, Nunn and other Democrats to come up with an alternative.

The Democratic leadership proposal will be considered as an alternative to the Weicker-Hatfield measure, which has been offered as an amendment to the defense authorization bill for next year.

Reagan has already threatened to veto the defense bill because of arms-control language that he opposes, and any war-powers restrictions would be further grounds for a veto.

Because Congress would have trouble overriding a veto, it is unlikely that any war-powers provisions will become law, although congressional action would serve as pressure on the administration to modify its policy. The pressure could also lead to compromise efforts to avoid the appearance of a divided government, in the view of some lawmakers.

In arguing against the Democratic proposal, Warner contended it would "fire a warning shot across every ship in the gulf" that the United States is uncertain about its commitment to the region. It could also impede U.S. efforts in the United Nations to put pressure on Iran to end its war with Iraq, including the tanker war in the gulf that led to the U.S. reflagging and escort operation, he said.

But Democrats contended that the nation is heading into a Vietnam-like, open-ended involvement in the hostilities that places U.S. escort forces directly in the line of Iran-Iraq fire. "By reducing the visibility of this kind of bull's eye operation, we hope to reduce the level of tension and begin to de-escalate the conflict in the gulf," Byrd said.

In addition to requiring congressional authorization within 90 days of enactment, the Democratic proposal calls for submission of a presidential report within 30 days, including objectives and progress in attaining them, estimated cost, diplomatic initiatives and anticipated duration of the operation. This is a relaxation of requirements of the existing law, which requires such a report within 48 hours and calls for congressional action within 60 to 90 days of the outbreak of hostilities.

In a related move, the Senate voted 91 to 4 for a non-binding "sense of the Senate" resolution that the Navy is "fully justified" in sinking any Iranian ship that threatens safe passage of U.S. warships or any vessels with Americans on board, which covers reflagged Kuwaiti tankers with U.S. captains. The measure was offered by Sen. Jesse Helms (R-N.C.).