Secretary of State Alexander M. Haig Jr. yesterday flatly rejected as "dead" the never-ratified U.S.-Soviet SALT II arms control treaty and warned Democrats they risk sowing "massive and instantaneous" controversy and confusion here, among allies and in Moscow if they continue to push for the abandoned pact.

Haig said that the Soviets, although not happy over the demise of the strategic arms limitation treaty (SALT II), "accept it and understand it." He urged lawmakers instead to "rally behind" President Reagan's newly announced proposal to begin talks with Moscow late in June that are aimed at much larger reductions in nuclear warheads and missiles of both countries.

Haig said the opening of the talks "might slip a few weeks," but he expressed no doubt that negotiations would begin.

Within hours of Haig's appearance yesterday morning before the Senate Foreign Relations Committee, Sen. John Glenn (D-Ohio) appeared before it to offer another plan for limiting nuclear weapons that calls for approval of precisely the ceilings negotiated in SALT II.

In recent months, several Democratic and a few Republican lawmakers, as well as officials of previous administrations, have called for revival of the 1979 strategic arms limitation treaty. The pact would have put an overall ceiling on missile launchers of both sides at roughly today's levels, required a reduction of about 250 older Soviet missiles (from the current 2,400-level) and also prevent adding still more multiple warheads to existing missiles.

Those who still support this treaty argue that the United States has nothing to lose by ratifying it and that if Reagan's new plan for strategic arms reduction talks (START) fails, this country will be left with nothing that constrains the Soviet arsenal. "Unless we proceed, and soon, to ratify the SALT II treaty . . . I think we face the very real prospect of a breakdown in the arms control process and another round of superfluous strategic weapons purchases," Glenn said.

But Haig was unbending on SALT's "fatal flaws." Most important, he said, the 1979 pact would have left Moscow with a big advantage in the form of hundreds of land-based missiles much larger than their U.S. counterparts and capable of knocking out those U.S. missiles in their protective silos.

It is these big Soviet weapons that the administration stresses are most destabilizing in terms of putting a hair-trigger on atomic war and that the administration seeks to limit severely in its new proposals.

In addition, Haig said SALT II would have left Moscow in a far better position than Washington to "break out" of the treaty, meaning the Soviets could quickly add many more warheads to their big missiles when the pact expired.

Although the Reagan administration's dislike of the Carter-era SALT II accord is no surprise, Haig's assertion that "we consider SALT II to be dead and have so informed the Soviet Union" appeared to rankle a number of senators. The treaty, though unlikely to be resurrected, is still technically before the Senate. Sen. Joseph R. Biden Jr. (D-Del.) told Haig "you may have told the Soviets but you haven't told us you want it the treaty back."

Sen. Paul S. Sarbanes (D-Md.) wanted to know when Moscow was told of SALT's abandonment. Haig said he didn't want to cite "one big day" but that this had been made clear in "countless discussions" with Ambassador Anatoliy Dobrynin and Foreign Minister Andrei Gromyko.

Haig stressed, however, that while SALT II as a treaty to be ratified was dead, the administration would continue to abide by its limitations as long as the Soviets did not violate them. He said there was no evidence that Moscow was not continuing to abide by the pact and that the Soviets had dismantled nine older missile submarines thus far in accordance with its provisions.

Haig also said as he has repeatedly before that the array of resolutions now before Congress calling for a nuclear freeze at existing levels of armaments would freeze in place Soviet advantages and remove incentive for Moscow to negotiate far more significant START cutbacks.

But later in the day, Sens. Edward M. Kennedy (D-Mass.) and Mark O. Hatfield (R-Ore.) argued before the committee that there was "essential parity" between the two superpowers and that the nuclear buildup must first stop before reductions can begin. The senators, co-sponsors of a resolution supported by 26 senators and 168 representatives, claimed that the Soviets will not stand still in their weapons programs while START goes ahead.

Under questioning about START, Haig said it was "unfortunate" that criticism by "certain American sources" was being used by Soviet news agencies but he said he thought Soviet leaders would "welcome resumption of these talks." Asked about a timetable or when results could be achieved, Haig said it would be "self-defeating to set any theoretical deadlines."

He said the United States would push for agreement as rapidly as possible on the first phase of the Reagan proposal, which is aimed at limiting warheads and land-based missiles. After that, an attempt would be made for a second agreement further reducing missile lifting power, or throw weight.

"These are tough proposals," Haig said, referring to the big reductions sought in the Soviet missile forces. "They clearly won't be easy for the Soviets to accept." In the meantime, Haig said it was vital to support President Reagan's plan to push ahead with new submarine missiles, cruise missiles and bombers.