President Reagan announced yesterday that he will seek Congress' approval to build 100 MX land-based missiles and 100 B1 bombers under a $180.3 billion program to upgrade the nation's nuclear forces.

The first 36 MXs would be placed in existing Titan missile silos, rather than hidden among new emplacements in Nevada and Utah as the Air Force had recommended.

Reagan and Defense Secretary Caspar W. Weinberger called these strategic initiatives stopgap measures to start closing the "window of vulnerability" that was an issue during the 1980 presidential campaign. A new generation of highly accurate Soviet missiles is said to have left existing U.S. land-based missiles much more vulnerable to attack than before.

Reagan also pledged to push ahead with development of the "Stealth," a radar-evading bomber for the 1990s; to put the silo-busting D5 missile aboard Trident 2 submarines as soon as it is ready late in the decade, and to improve military communications so they would work during a nuclear war.

To strengthen homeland defense, Reagan's new strategic blueprint calls for a modest but unspecified expansion of civil defense, continued research on ways to stop one missile with another, and making ICBMs or intercontinental ballistic missiles invulnerable to surprise attack. Burying them in mountains is one concept now up for study.

Reagan's rejection of the Air Force's recommendation to make the new MX mobile so it would be hard for enemy gunners to target is expected to draw the most fire from critics of this new strategic program. The Joint Chiefs of Staff may refuse to endorse that part of the package.

Weinberger conceded this could be the case, declaring "I'm not going to make any predictions" about the chiefs' support of the MX decision. "I have hopes," was all he would say.

No matter what the chiefs say when asked for their opinions on the strategic proposals during congressional hearings in the weeks ahead, Sen. John Tower (R-Tex.), chairman of the Senate Armed Services Committee, signaled trouble yesterday. He said he was "gravely disappointed" with the president's strategic decisions, especially the one to keep the MX standing still in vacated Titan silos.

Anticipating such criticism, defense officials at a Pentagon briefing yesterday emphasized that the MX-for-Titan switch would be just a quick fix, and that studies of other ways to deploy the giant missile would continue. For example, they said, the "Big Bird" airplane for carrying MX missiles is still a live option.

Where to go next with MX, if anywhere, will be decided in 1984, Weinberger said at a press conference held at the White House. Readying Titan silos in the meantime for the MX missile to come off the production line in 1986 would "buy time" to ponder future deployment options, he said.

The Air Force had hoped to deploy the MX around valley floors in Nevada or Utah, and preferably both, but the scheme set off a prairie fire of protests, including ones from the Mormon Church and area politicians.

Weinberger insisted yesterday that this political opposition was not what killed the idea of playing a shell game with 100 to 200 MX missiles by rotating them among 1,000 to 4,600 shelters. Instead, he said, the administration concluded that the Soviets could build enough warheads to cover all the shelters built for MX and blow them up with ease in a war.

Reagan then decided to trade mobility for fortification. Under his new plan, tons of concrete and steel will be piled around the 10-warhead MX to protect it against anything less than a direct hit from a one-megaton nuclear bomb. There are 52 operational Titan holes in Arizona, Arkansas and Kansas which could be rebuilt for MX. Defense officials did not reveal in which of these the first 36 MXs would be placed.

There was no indication where the balance of the 100 MXs would be deployed, although defense officials mentioned existing Minuteman silos in the West as a possibility. The MX could end up aboard an airplane if the "Big Bird" development work makes this look feasible.

Defense leaders in this and the past administration have said that the United States is confronted with a "window of vulnerability" in the mid-1980s because the Soviets have perfected enough accurate warheads to destroy the existing American force of land-based nuclear missiles, 1,000 Minuteman and 52 Titan ICBMs. The central question is whether Reagan's new strategic program will narrow or widen this "window of vulnerability."

The president said his decisions will "revitalize our strategic forces and maintain America's ability to keep the peace well into the next century." Ever budget conscious, Reagan anticipated critics of such expensive moves as building both the B1 and the advanced "Stealth" bomber by saying, "Let me point out here that this is a strategic program that America can afford."

Announcing his decision in the White House East Room, Reagan said it was made "after one of the most complex, thorough and carefully conducted processes in memory."

After reading his prepared statement, however, the president seemed ill-at-ease with questions about details of the decisions. He agreed to answer a few questions from reporters, but stressed that "for all technical matters, I am going to turn you over to Secretary Cap Weinberger."

Asked why the MX in fixed silos would be less vulnerable to Soviet attack than present missiles, he replied: "I don't know but what maybe you haven't gotten into the area that I'm going to turn over to the secretary of defense."

Weinberger, standing beside Reagan, quietly mentioned hardening the silos and then Reagan said, "The plan also includes the hardening of silos so that they are protected against nuclear attack."

When another reporter asked whether the B1 bomber would be able to penetrate Soviet air defenses, the president said: "I think that my few minutes are up and I'm going to turn that question over to Cap. I think I know the answer to it, but I do believe that you are getting into the kind of questions" that Weinberger was there to answer.

Reagan ran through the MX basing options trhat he had rejected, pointing out that one of them was to shuttle 200 missiles among 4,600 shelters as favored by the Carter administration. Another was to shuttle 100 missiles among 1,000 shelters which The Washington Post and other news organizations reported erroneously earlier this week as Reagan's choice.

The president said that his decisions "will signal our resolve to maintain the strategic balance and this is the keystone to any genuine arms reduction agreement with the Soviets."

A number of reporters asked Weinberger whether the Reagan package with its considerable enhancement of the U.S. nuclear arsenal indicates that the administration believes a nuclear war can be fought and won.

Weinberger refused to be pinned down. "I think the way in which our success can be measured is if we never have to use any of these things," he said.

"The point I think is not whether we think we can win a nuclear war. It is that unless we acquire the means to deter a nuclear war we may very well be subjected either to a nuclear attack or to the kind of intimidating force that could produce the subjugation of the country," he said.

Reagan and Weinberger say that what is important is that the Soviets think a nuclear war can be won.

Here are other specifics of President Reagan's strategic proposals:

MX missile. The missile itself is already under development and scheduled to be ready for deployment in 1986. The first 36 will go into rebuilt Titan silos as soon as possible. Defense officials hope to have the full 100 deployed by 1989.

New bombers. The first squadron of 14 B1 bombers would be flying in 1986, while the "Stealth" would be ready for duty in the 1990s under the Pentagon's timetable. Defense officials would give no cost estimates, but the fleet of 100 B1s is likely to cost about $25 billion. "Stealth" research and development is running at more than $1 billion a year and rising.

"The B1 will be able to penetrate Soviet defenses initially and will make a good cruise missile carrier and conventional bomber after the Advanced Technology Bomber (Stealth) is deployed and all B52s are retired in the 1990s," the administration said in its 10-page description of its strategic program distributed at the White House and Pentagon.

D5 submarine missile. This is a more accurate and lethal version of the Trident I missile to be ready in 1989. It could be used to destroy Soviet missiles in their silos, giving it a "first strike" potential as distinguished from the early Polaris submarine missiles which were developed principally for retaliatory strikes against soft targets like cities. The administration also said it intends to build one new Trident submarine a year for the foreseeable future, but how many it intends to build overall it did not say. And in a new departure it said it would now put nuclear-tipped cruise missiles aboard smaller attack submarines. These were already slated to be equipped with cruise missiles, but with conventional warheads.

Air defense. "At least" six additional AWACS (airborne warning and control systems) planes and five squadrons of F15 fighters will be be purchased to defend the continental United States.

Ballistic missile defense. This would be continued as a research effort because defense officials concluded it was not leak-proof and thus could not be relied upon to defend MX and other land missiles. The Pentagon also will explore the feasibility of basing lasers on space ships to fire at any incoming enemy warheads. But this is considered "blue sky" research, with little prospect of any such weapon being perfected soon.