Moderates in Congress who supported the MX missile earlier this year on condition that the Air Force turn to a smaller, less threatening mobile missile in the future are moving now to make sure the White House and Air Force keep that promise.

Dismayed by some indications that the smaller Midgetman missile already is growing heavier and less mobile than first envisioned, the moderates now want to vote a weight limitation into law.

For fear that the administration may not be wholeheartedly committed to Midgetman, the moderates also are seeking to prevent deployment of more than about 30 MX missiles until the Midgetman has been flight-tested or has passed some other concrete milestone.

Pentagon officials, while assuring congressional doubters that they are "very enthusiastic" about Midgetman, nonetheless are opposing attempts to restrict deployment of the MX.

"We're in a big fight," said Rep. Les Aspin (D-Wis.), who is leading the move to place conditions on further MX funds.

Many moderates say they believe the Midgetman would be more conducive to arms stability than would the giant MX because it would be both less powerful and a less tempting target.

Some say they fear, however, that the administration does not believe in the concept of a single-warhead missile, which would depart from past trends of building ever more powerful land-based missiles, and only agreed to develop it in order to win votes for the MX with its 10 nuclear warheads.

The House of Representatives is expected to vote later this month on a defense authorization bill that provides $2.5 billion to purchase the first of a planned 100 MX missiles. A second vote to appropriate the funds is expected in the fall.

Although supporters and opponents of the controversial missile say the House is likely to continue support for the MX in both votes, neither side considers the outcome anything close to a sure thing. As a result, the votes of Aspin and other moderate Democrats who surprised some people by supporting the MX in May could be crucial again.

Pentagon officials, in an effort to avoid restrictions as much as possible, yesterday had Air Force Brig. Gen. Gordon Fornell brief reporters on the progress of the Midgetman program.

Fornell acknowledged that the idea of a small mobile missile initially caused "a little bit of a cultural shock for us," but said the Air Force now regards Midgetman as "a magnificent opportunity" to explore new technology.

Fornell also said Secretary of Defense Caspar W. Weinberger is solidly behind the program and is leaning toward a "hard mobile" scheme of deployment. That plan calls for Midgetman missiles to be stored and, in times of crisis, driven around western military reservations on trucks that would be "hardened" to survive anything but a direct nuclear attack.

Fornell said the Air Force plans to award initial contracts on Midgetman this summer. Those awards would seek to define the goals and likely features of the weapons program, with full development beginning in 1987 and deployment in the early 1990s.

The program is now estimated to cost about $600 million each year until 1987, more than $1 billion each year for further development and about $70 billion during a 10-year period of operation.

"I think that clearly demonstrates the excitement, the enthusiasm, by absolute dedication of resources and dollars, toward activating that program," Fornell said.

The Midgetman received its major boost from the president's Scowcroft commission on strategic issues, which recommended placing 100 MX missiles in existing Minuteman silos but acknowledged that they would be vulnerable to attack from increasingly accurate Soviet missiles.

In the long run, the commission said, the United States should move toward single-warhead, mobile missiles that would present less of a first-strike threat to the Soviets and at the same time would be less vulnerable to a first strike.

The administration accepted the report, which said Midgetman should weigh about 30,000 pounds to preserve its mobility. Some Pentagon officials, however, began suggesting that the missile might have to weigh almost 40,000 pounds to reach Soviet targets from the Southwest.

Aspin and others began to fear that Midgetman would grow into a stationary "Tubbyman" missile with vulnerability problems like the MX, according to an aide, and sought to write a 30,000-pound limit into law. That brought Aspin a visit from Pentagon research chief Richard D. DeLauer, who said a legal limit would unduly hamper flexibility.

Fornell said yesterday that the Pentagon is committed to keeping the missile as light as possible. "It absolutely can be mobile," he said. "We will preserve the mobile option."