Supporters of President Reagan's decision to deploy MX missiles yesterday defended his Wyoming basing plan in terms of its deterrence value while critics attacked it in terms of its death threat. A Pentagon spokesman predicted that Congress will approve the plan.

Several strong believers in the need to have the MX missiles themselves were only lukewarm on the "Dense Pack" plan for locating them close together on a narrow strip of land at Warren Air Force Base, which is just outside of Cheyenne. They wondered mainly if the idea would work as advertised to deter the Soviet Union from striking first.

Proponents of a nuclear freeze focused instead on the mind-bending horrors of nuclear war, arguing that the Soviet Union will see the MX missile as an escalation of U.S. ability to strike first and will therefore accelerate their nuclear buildup, making everyone more nervous and bringing war that much closer.

All sides, except for the Defense Department, predicted a tough fight in Congress, which is thought to be closely divided on the subject and has 30 legislative days to decide on the plan.

Some kind of early vote on it is expected during the lame-duck session that begins Monday.

Pentagon spokesman Henry Catto, going further than Defense Secretary Caspar W. Weinberger did, told reporters at a briefing that the plan would pass Congress.

"This is what we are going to get," he said. Weinberger had earlier refused to make any prediction, saying only that he was hopeful it would pass.

Sen. John G. Tower (R-Tex.), chairman of the Senate Armed Services Committee and a strong supporter of MX development, said he was "cautiously hopeful" that the Dense Pack scheme "will provide the desired measure of survivability" for land-based missiles that MX backers say is the issue.

"Technical and programmatic issues involved in this system remain to be assessed," Tower said. "I intend to keep an open mind."

Wyoming Gov. Ed Herschler, a Democrat, met with Reagan for 15 minutes but did not reveal what was said.

On Monday, Herschler told western governors in Denver that he supported the MX placement in his state although he thought it was both good and bad, "like a mother-in-law driving your new Cadillac over a cliff, or a teen-age daughter coming home at 3 a.m. with a Gideon Bible under her arm."

Sen. Alan K. Simpson (R-Wyo.), noting that he had sought neither to have the MX based in Wyoming nor to keep it out, promised extensive hearings on "all the possible impact that can come into the state" with the missiles.

Sen. Ernest F. Hollings (D-S.C.) has all but guaranteed a congressional battle with promises that when crucial funding bills come up during the lame-duck session, he will offer amendments to kill development money for the first six MX missiles.

He came up four votes short in such an effort during the last session.

MX critics said they will work to stir grass-roots opposition that could close that gap, pushing for a congressional call for a verifiable, bilateral freeze on deployment of nuclear weapons.

Joining in a renewed call for the freeze were Sens. Edward M. Kennedy (D-Mass.) and Mark O. Hatfield (R-Ore.), and Reps. Edward J. Markey (D-Mass.) and Jonathan B. Bingham (D-N.Y.).

The Soviets might accept such a call, they said, but would be alarmed by MX deployment.

"If Russia did what we now propose to do, we would clearly think of it as a violation" of the strategic arms limitation talks agreements that prohibit new fixed missile launchers, said former chief SALT negotiator Paul Warnke at a news conference of MX opponents.

Retired Adm. Noel Gayler, former director of the National Security Agency, agreed, saying the Soviets "are going to match the MX if it takes their last kopek and they're going to feel more insecure than they do now."