The Defense Department is not planning to shield the Midgetman missile from this month's round of budget cuts, a sign of the department's increasing skepticism about the value of the proposed mobile nuclear weapon and the future of land-based missiles, officials said this week.

The Reagan administration has consistently protected its strategic nuclear program from congressional budget cutters more strenuously than other weapons, and the MX missile and Stealth bomber will both be shielded this year from Gramm-Rudman-Hollings deficit reductions. But Midgetman, a favorite of some arms control advocates in Congress, will be permitted to take its lumps with most of the rest of the budget, probably a reduction of about 5 percent from what the Pentagon currently desires.

Officials inside and outside the administration said the decision does not indicate that the Pentagon has given up on Midgetman, which many experts see as the administration's last hope to add substantially to the land-based nuclear force. A Pentagon-commissioned study by outside defense experts, which has not been released, is expected to endorse Midgetman, officials said.

But senior officials in the Defense Department have cooled considerably in their ardor for Midgetman, particularly as shrinking budgets may force it to compete for funds with President Reagan's Strategic Defense Initiative, or "Star Wars" research program to parry nuclear missiles. One official said this week that the administration, which came into office decrying the "window of vulnerability" caused by unprotected land-based missiles, may now have to be satisfied with beefing up its "aerodynamic" force -- bombers and cruise missiles.

"I think it's just a question of expense," another senior official said of the Midgetman. "I'd take them, but it's going to cost $50 billion or $60 billion, and that will come from other things. I would much rather spend the money on missile defense."

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee and a Midgetman supporter, said the Pentagon's waning enthusiasm for Midgetman reflects the general disarray of its strategic policy. He said the administration appears to be backing away from the Scowcroft Commission report of 1983, which called for 100 MX missiles -- 50 of which Congress has funded -- as well as a force of small mobile missiles that theoretically would be difficult for Soviet missiles to destroy in a sneak attack.

"My sense is the administration's whole strategic program is in chaos," Aspin said. "They don't know whether they're coming or going, and they're going to end up where they were before the Scowcroft Commission, without any land-based missile force."

Aspin noted that Midgetman's demise would not only release dollars for the Star Wars missile defense program but would also boost the rationale for it, since advocates could argue the need for defenses around MX silos.

The Air Force, which originally had little enthusiasm for Midgetman, has become more of a booster as the number of people and dollars devoted to the program has grown. Yesterday the Pentagon sent Congress an Air Force study comparing the effectiveness of small missiles at various potential weights.

"For the program guys, it's their baby," said a congressional aide. "Even if you think your baby is ugly at first, eventually you learn to love it."

The Pentagon is preparing budget cuts required by the Gramm-Rudman-Hollings law, which triggers automatic cuts when the budget deficit exceeds a certain figure. For fiscal 1986, which ends Sept. 30, Congress gave the Pentagon flexibility to exempt programs from automatic cuts if other programs in the same general area are cut deeper to make up the difference.

Midgetman will not be cut sharply but it will not be protected, either, officials said, and its prospects in the budget of fiscal 1987, when the Pentagon may face huge cuts in excess of $50 billion, are uncertain.

The Small Intercontinental Ballistic Missile, as Midgetman is officially known, is meant to reduce the vulnerability of land-based missiles by virtue of its mobility. Soviet missiles would not be able to target Midgetmen, its backers say, because it would roam on government land in the western United States on trucks or tracked vehicles.

The ostensible vulnerability of fixed-silo missiles, such as the existing Minuteman force and the 10-warhead MX, has led Congress to refuse to fund more than 50 MX missiles. A proposal to "super-harden" silos against nuclear blast is now thought to hold little promise, one official said, because the Soviets will respond by improving the accuracy of their warheads.

A Pentagon panel established to review the Midgetman program, headed by Massachusetts Institute of Technology provost John Deutch -- who also served on the Scowcroft panel -- is expected to endorse Midgetman as well as "carry-hard" basing for MX. Carry-hard would combine mobility with super-hardening, calling for missiles to be transported among a number of silos in blast-hardened cases.