Secretary of State George P. Shultz and Secretary of Defense Caspar W. Weinberger took the administration's new case for the MX missile to Congress yesterday, with Shultz saying it "is of critical importance to our foreign policy" and Weinberger saying that without it Soviet policy will become "bolder and more obnoxious."

As the Cabinet officers appeared before the Senate Armed Services Committee, three more key senators came out in support of President Reagan's controversial plan to base 100 MX missiles in existing Minuteman missile silos.

But Shultz and Weinberger also met with continued sharp skepticism from several other lawmakers, including Sen. Edward M. Kennedy (D-Mass.), who said he wondered why deployment of new missiles that would be "sitting ducks" in admittedly vulnerable old silos would convince Moscow of American resolve.

Sen. Carl Levin (D-Mich.) said such a plan "turns logic on its head. It is just the opposite of deterrence."

Committee Chairman John G. Tower (R-Tex.), one who came out for the plan yesterday, did so in a way that captured the frustration and lack of enthusiasm for it even among those who support MX.

"If anything," Tower said, referring to the 10-year history of indecision on how to keep the MX safe from Soviet attack, "the MX program is a textbook case of how not to manage an important national security issue."

"However," he said, "present circumstances being what they are, I feel compelled to concur in the recommendations" last week of the presidential advisory commission on arms, which Reagan formally adopted Tuesday. Later, Tower told reporters that, given all the political opposition to previous basing ideas, installing the missiles quickly into the existing silos "is the best we can get."

As for many of those opposing the current basing plan, Tower said, "A lot of those people who make those arguments don't want MX under any solution."

Shultz and Weinberger argued forcefully that the MX is essential if the United States is to deal with Moscow from a position of strength, including in arms-control talks. They said the MX would discourage the Soviets from thinking they had an advantage in nuclear striking power and from running "risks in a regional conflict or crisis."

Shultz also argued that the decision to deploy the MX here would be watched closely by allies in Europe who are being asked to accept other new and controversial U.S. missiles on their soil.

In recommending deployment of the new missiles in the Minuteman silos, the special commission last week acknowledged that those silos are vulnerable to a Soviet first strike.

But the commission also overturned a decade of Pentagon dogma, including warnings by Weinberger and Reagan, by claiming that the vulnerability is not as serious as previously held because the Soviets can not attack the entire U.S. land-, sea- and air-based missile force at the same time.

It is that sharp turn of the argument that has caused so much skepticism. "We are basically rejecting 85 percent of all the testimony" made in congressional hearings on the question of how to make the MX survivable, said Sen. J. James Exon (D-Neb.).

But Sen. Henry M. Jackson (D-Wash.) said he does not agree that the MX decision was purely political, and stressed that it is time to "get something done." Jackson suggested, however, that 75 missiles may be enough to convince Moscow that the United States is determined to modernize its force.

Sen. John C. Stennis (D-Miss.) added his support, ensuring a that healthy majority of the Republican-run committee will vote for the missile.

Immediate jurisdiction over the $16.6 billion program, however, rests with the Appropriations committees. The leader of the Senate Appropriations Committee, Sen. Mark O. Hatfield (R-Ore.), is strongly opposed, as is Rep. Joseph P. Addabbo (D-N.Y.), who heads the House Appropriations subcommittee on defense.

Sens. John W. Warner (R-Va.) and William S. Cohen (R-Maine.) asked Shultz and Weinberger why the Soviets might not conclude that the 100 new missiles, each packing 10 warheads, meant the United States is striving for a first-strike capability.

Shultz said 100 missiles are not enough for that. But Cohen and Warner pointed out that the United States also is building hundreds of Trident II submarine missiles, which will have the same ability as the MX to knock out Soviet silos. Weinberger said Trident II will not be available until 1989 and that it will take even longer for substantial numbers to enter the fleet.

Sen. Sam Nunn (D-Ga.), who supports the new Reagan plan, also voiced concern that the United States and the Soviet Union "are inexorably moving towards launching their missiles much more quickly than has been the case in the past," and that such a policy of launch-on-warning could be extremely dangerous. The administration has denied that it is moving toward such a policy.

Kennedy and Sen. Jeff Bingaman (D-N.M.) pointed out that aside from the Trident the United States also is building 3,000 air-launched cruise missiles, 500 new sea-launched cruise missiles and two new bombers and is deploying 572 new missiles in Europe.

Weinberger answered that those weapons will not enhance the United States' ability to deter nuclear attack as much as would the MX, which has the capablity to strike back retaliate quickly and accurately at well-protected Soviet targets and to balance the threat that Soviet missiles now present to the U.S. force.