This week Congress will take its first critical look at the newly controversial small mobile missile called Midgetman.

It is seen by many as a solution to America's strategic problems, but has become the subject of serious second thoughts because of new developments that raise doubts about its benefits.

Hatched three years ago by a presidential commission appointed to salvage President Reagan's strategic modernization program, Midgetman could be the land-based intercontinental ballistic missile of the future, or it could blow up politically on the launching pad, like the giant MX.

Until now Congress has embraced the Midgetman as a kind of intercontinental security blanket while it pushed away the MX. Midgetman, it was argued, would be so mobile that Soviet missiles, no matter how accurate, could not destroy them in a surprise attack. The MX, by contrast, will stand in fixed missile silos, making them relatively easy for the Soviets to hit. Midgetman enthusiasts have conceded from the start that the Soviets could deploy enough warheads to lay down a barrage of nuclear explosions over Midgetmen bases that would be sufficient to destroy them, even if they were kept moving.

But, the proponents argued, Reagan through arms control negotiations would reduce the number of warheads the United States and Soviet Union had aimed at each other. Also, the planned 100 silo-busting MX missiles to be trained on Soviet SS18 and SS19 ICBMs would convince the Kremlin that no amount of steel and concrete "hardening" could protect those heavyweights of the arsenal. The Soviet planners, like their U.S. counterparts, would conclude that the only way to save land missiles was to keep them moving.

Backers of Midgetman said the Soviets might see its merits for their arsenal and begin moving away from hair-trigger multiwarhead missiles to single-warhead ones that are obviously less useful as first-strike weapons than as retaliators.

That world of Midgetman envisioned by the presidential commission, headed by retired Air Force general Brent Scowcroft, has changed vastly in just three years. Senior members of the Senate and House Armed Services committees, according to several, will come to grips with these changes for the first time when they meet to debate rival defense authorization bills for fiscal 1986.

These developments have changed the outlook:

*Reagan not only has so far failed to negotiate U.S.-Soviet reductions in warheads, but encountered criticism from the right wing when he decided to stick with the modest limits on missile launchers in the SALT II pact negotiated by President Jimmy Carter. But without strict limits on Soviet warheads, Midgetman will be threatened with more warheads than it could escape.

*Congress has refused to approve the deployment of 100 MX missiles in existing silos; the Senate has voted a "pause" after the deployment of 50, and the House approved "capping" the force at 40. This saws in half the club the Scowcroft panel thought would convince the Soviets to give up on their silo-based giants which could hurl a shower of warheads at a Midgetman base.

*Reagan has launched the Strategic Defense Initiative, or "Star Wars," which is designed to destroy Soviet missiles right after launch or stop their incoming warheads. The easiest short-term Soviet response to SDI would be to deploy more warheads so as to overwhelm it, raising the question of whether the single-warhead Midgetman could survive in such a nuclear war.

Midgetman and the rest of Reagan's strategic blueprint have been torn by these and other developments since he adopted the Scowcroft commission recommendations of 1983, according to Sen. Pete Wilson (R-Calif.), who will be among those at the House-Senate conference demanding a fresh appraisal of the programs. "President Reagan's once vaunted strategic modernization program must now be reassessed and revised," Wilson said.

He said that Congress has pushed so much money on the Air Force to hurry up Midgetman that the missile ought to be called "Congressman." He said Midgetman, at 15 tons, will not be big enough to carry more than one warhead and the devices needed to penetrate the expected Soviet defenses of the future.

Sen. Sam Nunn (D-Ga.), ranking Democrat on the Armed Services Committee, said he, too, is switching his focus from MX to Midgetman. He said nobody knows how much the missile will cost or how many people will be needed to operate and guard it, or whether it would be cheaper and wiser to put more missiles to sea in submarines rather than continue to pursue invulnerability for land-based ICBMs.

And Sen. John Glenn (D-Ohio) concedes that proliferating Soviet warheads might be able to smother Midgetman if the missile is confined to military bases, as the Pentagon envisions. The answer is to put the missiles on the road all over the United States in trucks which look no more lethal to Soviet spying methods than market delivery vans.

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, rejects the gloomy talk about Midgetman. He said that the Scowcoft commission came close to recommending 50 MXs as sufficient to persuade the Soviets to relinquish their silo-based, multiple-warhead ICBMs. Therefore, the congressional cap does not dim Midgetman's future, he said.

Also, Aspin said, the Soviets already are moving to deploy mobile ICBMs, which can induce the United States to try to match them. And if negotiations fail to reduce the number of warheads, the United States could place missile defense systems around Midgetman bases to change the odds.

Rather than regard SDI as provoking the Soviets into deploying more warheads, Aspin described it as the blue chip in any poker game U.S. arms control negotiators play with the Soviets. Aspin said Reagan could agree to forgo deploying the new missile defense if the Soviets would reduce the size of their missiles' potential payload and number of warheads.

The Air Force headquarters for Midgetman seems serene despite this growing debate. Colonels there said Midgetman, which they call SICBM, for small intercontinental ballistic missile, is coming along just fine. Models of armored vehicles designed to haul the missile appeared to survive a simulated nuclear explosion at the White Sands, N.M., test range on June 28.

Air Force officials said they could provide no cost estimates for the total Midgetman program or say how many people would have to operate and guard the missile because the program's size has not been defined. Midgetman critic Sen. Wilson said the program could cost $100 billion and require 50,000 people if 500 to 1,000 missiles were deployed.