President Reagan proposed to Congress yesterday the deployment of 100 MX missiles in the closely spaced "Dense Pack" formation at Warren Air Force Base in Cheyenne, Wyo. He estimated his plan would cost $26.4 billion.

If the Soviets figure out a way to destroy the MX, the president said he might protect the missile field with anti-ballistic missiles or dig more holes, still only at Warren AFB.

Renaming the MX "the Peacekeeper," Reagan in a letter to Congress said he hoped to have the first missiles on line at Warren "late in 1986." The 100 missiles would be deployed in a column of silos 1,800 feet apart on the theory that such tight spacing would cause incoming Soviet warheads to knock each other out if they went after the MX.

Reagan, who had ridiculed President Carter's plan to spread 200 MX missiles far apart on the valley floors of Nevada and Utah, said last night he decided to go ahead with the MX because "we must replace and modernize our forces."

The president did not claim the MX would close a "window of vulnerability" he deplored during his 1980 election campaign. Reagan and his advisers contend the Soviets opened that window by deploying warheads on their new missiles that are accurate and powerful enough to knock out 90 percent of the current U.S. force of 1,000 Minuteman and 51 Titan land missiles.

The idea behind Dense Pack, as in earlier MX deployment schemes, is to make Soviet doomsday planners conclude that there is no way to knock out most of the U.S. force of land missiles in a surprise strike. Just 13 months ago Reagan had recommended putting the first MX missiles in existing Minuteman and Titan silos, an idea that Congress rejected on grounds it would be just submitting a new missile to the same old risk of being destroyed before it could be fired.

Reagan said the Dense Pack formation would require half as many MXs as the Carter "drag strip" plan and "will fit in an area of only 20 square miles." Pentagon leaders said that any additions to the MX deployment would be confined to Warren AFB, not other areas the Air Force had surveyed.

Defense Secretary Caspar W. Weinberger told a Pentagon news conference that Dense Pack will give the U.S. land missile force "survivability well into the 1990s." His deputies at the Pentagon said the cost of deploying 100 MXs at Warren would be $26.4 billion. It would cost another $9 billion to $12 billion to protect the MX field with the Spartan and Sprint anti-ballistic missiles developed two decades ago and $2 billion to $3 billion additional to dig 200 more holes at Warren to confuse Soviet gunners even further.

Weinberger and his research deputies, who could not be identified under the ground rules of yesterday's Pentagon briefing, rested their case for Dense Pack on the contention that it would drive Soviet doomsday planners back to the drawing board.

Now that the Soviets have finally perfected warheads accurate, powerful and numerous enough to crack Minuteman and Titan silos, so the missiles inside could not get out, the Dense Pack bunched formation would require them to change course in building nuclear warheads, according to the Pentagon.

As the nuclear balance stands now, said an Air Force research chief, the Soviets could knock out existing U.S. land missiles by firing off only one-fourth of their ICBMs. The remaining Soviet rocket force, the Pentagon contends, could be used for gun-to-the-head negotiations with the American president.

But with Dense Pack, went the Pentagon's argument, the Soviets would have to commit so many warheads to the MX field that they could not cover the existing Minuteman formations spread around the Northwest at the same time.

Although building more warheads sounds like the quick fix to that coverage problem, Dense Pack advocates said this is not the case. The first warhead that exploded would destroy the others in the tight formation it would take to crunch MX silos by exploding in the air over them.

Rather than try to develop warheads that would explode all at once over the MX runway, which would be 14 miles long and 1 1/2 miles wide, Pentagon research chiefs predicted the Soviets would give up on that and switch to building 25-megaton warheads for their SS18 missiles. Those warheads would be designed to explode on the ground, not overhead, to dig the MX missiles right out of their silos.

But that poses another set of problems, the specialists say. Such explosions on the ground throw up X-rays and other deadly rays, create a fireball and kick up tons of debris -- all of which can destroy the blockbuster warheads coming in behind the first wave. And there will be so much concrete and steel around each MX capsule, the Pentagon briefers said, that it would take one blockbuster to destroy each of the 100 missiles.

To try to overcome the nuclear "fratricide" of air and surface explosions, an Air Force research chief predicted the Soviets would try to develop warheads that would bury themselves in the ground around the MX missiles and explode all at once like land mines. But this will take some doing, he continued, buying the U.S. land missile at least 10 years' grace before having to resort to such other measures as ABM protection or additional MX silos near the first 100 at Warren.

Critics say the United States does not need a new land missile; that the Soviets already know that attacking existing Minuteman and Titan missiles would be suicidal since it would bring down on them a rain of nuclear warheads launched by American bombers and submarines. Weinberger said yesterday that this was not enough deterrence.

"We need the redundancy that is provided by all three," he said, referring to the strategic nuclear "triad" -- land missiles, bombers and submarine missiles.

The MX is designed to carry 10 warheads of about 335 kilotons each which would be both powerful and accurate enough to crunch Soviet missiles protected by tons of concrete and steel.

This makes it a counterforce weapon that could be used in surprise attacks against Soviet missile fields. The Pentagon contends that the Soviets already have such a counterforce weapon in the SS18 and the United States must fill that gap with a survivable MX.