Congressional Democratic leaders warned yesterday that they will pressure President Reagan to stay within the limits of the SALT II arms control agreement and that his decision to abandon the unratified treaty later this year could further jeopardize funding for his favorite Pentagon program, the Strategic Defense Initiative.

Sen. J. James Exon (D-Neb.), ranking Democrat on the Senate Armed Services subcommittee that will approve the budget of the missile defense program, said he "may have to waver in my generally strong support of SDI" if as a result of the president's statement last week both superpowers ignore the SALT II limits "and the arms race takes off on a new fast track."

Bipartisan groups in the Senate and the House said they would draft proposals to restrict the administration from spending more for missiles than would be allowed by the SALT II treaty, which was signed in 1979 but never ratified by the Senate.

Meanwhile, U.S. allies continued to object strongly to both the substance of Reagan's decision and the manner in which they were informed of it. The allies were told in advance about Reagan's two-edged decision, which called for dismantling two missile-firing submarines to keep the United States within the SALT II limits for the present. They also were told that the United States was likely to exceed the limits late in the year when more B52 bombers are equipped with air-launched cruise missiles.

However, this advance notification by ambassadors Paul H. Nitze and Edward Rowney did not include the information that Reagan would make a complete break with the principles of SALT II, according to diplomatic sources.

"We are now facing a six-month internal debate within the alliance," said a high-ranking NATO diplomat yesterday.

The Soviets, meanwhile, sought to gain advantage from the dispute in world opinion by asking the Western allies to seek U.S. adherence to SALT II. Soviet President Andrei Gromyko met with members of the British Parliament and asked them to try to influence the United States to keep SALT II. Reagan has said Moscow systematically violates the accord.

A delegation member quoted Gromyko as saying, "SALT II has been dealt blow after blow by Washington. This latest action is a high explosive charge under it. This is a major American blunder."

White House spokesman Edward P. Djerejian yesterday repeated U.S. charges that Soviet violations have discredited SALT II. He complained that major newspapers gave "very little coverage" to White House reaction to Soviet statements denouncing the president's decision.

But in comments later in the day Djerejian and other White House officials left the door open to a change in the president's position this fall if the Soviets correct treaty violations and negotiate "seriously" on arms control proposals being considered in Geneva.

"The Soviet Union has another opportunity here to give us pause to rethink our position," Djerejian said. "It depends on what they put on the table in Geneva. We've gone the extra mile, but there's a few more inches in it."

White House legislative liaison William Ball, saying the administration was aware of some congressional discontent with the decision, said the president will be "monitoring Soviet behavior and reacting to it."

Congressional attempts to force the administration to remain in compliance with SALT II could face constitutional problems because the treaty was never ratified, according to both congressional and White House sources. Congressional sources said they hope to overcome these problems with an amendment to the defense authorization bill.

Sen. Edward M. Kennedy (D-Mass.), yesterday denounced Reagan's decision and called it "a triumph of ideology over common sense." He said that if the administration was engaging in a "ploy to wring greater concessions from the Soviet Union and enhance the prospects" for a successful summit, it is playing "a dangerous game."

Administration officials disputed the contention of Reagan critics that the abandonment of SALT II would intensify the arms race. Djerejian said the Soviets have sufficient nuclear missiles and are constrained by economic conditions. Secretary of State George P. Shultz said Sunday on NBC-TV's "Meet the Press" that he did not anticipate "any appreciable numerical growth in U.S. strategic forces."