Key members of Congress yesterday sharply criticized one feature of the new U.S. arms plan submitted to the Soviet Union: a ban on mobile missiles that would scuttle the proposed Air Force Midgetman if Moscow abandons its land-based mobile missiles.

Other aspects of the new U.S. proposal are expected to draw fire in Moscow next week when Secretary of State George P. Shultz discusses it with Soviet leader Mikhail Gorbachev. But the bid to ban mobile missiles provoked immediate objections on Capitol Hill, where two influential supporters of the Midgetman said it undermines a fragile political consensus forged in Congress over the last two years.

"It's a bad idea," said Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, who helped win House approval of 50 MX missiles in return for an administration pledge to pursue the mobile Midgetman.

Sen. Albert Gore Jr. (D-Tenn.), another architect of the MX-Midgetman compromise, said the idea of a ban on mobile missiles is "deeply disturbing. I hope the president would reconsider it in advance of Geneva."

Gore, Aspin and other supporters of the Midgetman think a mobile, land-based missile that the Soviets could not easily target is a crucial ingredient to a stable balance of nuclear forces, a position originally endorsed by the Reagan administration when it was first enunciated by the Scowcroft Commission, which was named by President Reagan to analyze U.S. strategic needs.

The decision to call for a ban on mobile missiles, which was controversial in the executive branch, was among the last made by Reagan in shaping the U.S. proposal, an administration official said last night.

The official listed four reasons that persuaded Reagan to endorse the proposed ban: The difficulty of verifying mobile missiles in any arms control agreement; the vast territory available to Soviet military planners seeking to deploy such missiles; the fact that the Soviet mobile SS24 and SS25 missiles are much more advanced than Midgetman, which is still on the drawing board, and the uncertainty of congressional support for the high-priced Midgetman.

The U.S. proposal that Shultz will outline to Gorbachev next Monday and Tuesday embodies the general idea of a 50 percent cut in nuclear arms -- proposed last month by the Soviets -- in ways considered more equitable by the Reagan administration, U.S. officials said.

The new U.S. proposals offer several new carrots to the Soviet Union but generally represent modifications of earlier U.S. suggestions that the Soviets have rejected.

Many of the elements of the new U.S. plan are likely to be unpalatable to Moscow, administration officials said, but they expressed hope that some aspects of the new offer would appeal to the Soviets, just as some aspects of the Soviet proposals were appealing to the United States.

In Moscow, the Soviet news agency Tass said the proposal consists of old ideas that have been "slightly modified and presented in a new wrapping."

The new U.S. offer conspicuously omits any substantial concession to Moscow on the limitation of research, testing, development or deployment of space-based antimissile systems such as those envisioned in Reagan's Strategic Defense Initiative (SDI), or "Star Wars" plan. The Soviet Union has made limitations on space weaponry its top priority in the Geneva negotiations and a condition of large-scale cuts in offensive weapons.

One member of Congress said yesterday he had been told by an administration official that if the United States decides to offer any concessions on Star Wars development, they would be withheld until much later in a negotiating process that is only expected to begin at the summit meeting in Geneva Nov. 19 and 20.

These are key aspects of the new U.S. offer, according to administration officials:In U.S. calculation, the Soviets have about 9,000 ballistic missile warheads, compared to about 8,000 U.S. ballistic missile warheads. The new U.S. proposal calls for a common ceiling of 4,500 ballistic missile warheads, half the current Soviet total. (The earlier U.S. proposal included a ceiling of 5,000 warheads.) The United States calculates that the Soviets have 6,400 land-based ballistic missile warheads, the most powerful and accurate kind. The new U.S. proposal would cut this a little more than half, to 3,000 land-based warheads. This is less than the limit of 3,600 land-based warheads proposed by the Soviets in their recent offer, but more than the 2,500 previously proposed by Washington.

The United States currently has deployed 2,118 warheads on land-based missiles aimed at the Soviet Union, and 5,536 warheads on submarine-based missiles. So to meet the overall limit of 4,500 total warheads in the new American proposal, the United States would have to cut substantially from its existing force of submarine missiles. That prospect raises a potentially significant carrot for the Soviets -- reduced deployment of the next generation of sub-based missiles known as D5s. The D5 is intended to be a super-accurate missile that could carry seven or eight warheads each, and the Soviets would presumably be interested in minimizing its deployment.

*The United States, according to its estimate, has 570 long-range bombers under strategic arms accounting rules (though less than half of these are in operation). The planners of the U.S. proposal originally suggested that this be cut roughly in half, to 280 long-range bombers. The Joint Chiefs of Staff, however, opposed so low a total in an area of comparative U.S. advantage. The final limit in the U.S. proposal is 350 bombers, still less than the limit of 400 proposed by Washington in the past.

*The Defense Department's plans for air-launched cruise missiles to be placed aboard long-range bombers calls for eventual production of 3,000 missiles, according to administration officials. In its new proposal the United States applied the 50 percent formula and is now proposing a limit of 1,500 air-launched cruise missiles, which are pilotless drones directed to their targets by on-board computers. (The Soviets propose to ban all long-range cruise missiles; previously the United States offered a limit of 4,000).

*The U.S. calculation of Soviet strategic missile "throw weight," which is a measurement of the potential payload the missiles can carry, is 6 million kilograms. The U.S. proposal calls for a "throw-weight" limit of 3 million kilograms, half of the Soviet total and close to the actual total for U.S. strategic missiles.

*Concerning intermediate-range nuclear forces (INF) -- missiles based on both sides of the Iron Curtain in Europe whose range makes them useful only against European targets -- the U.S. proposal calls for a freeze, to be followed by further negotiations, just as the latest Soviet proposal does. However, the United States proposes a subsequent reduction to 140 launchers on each side in Europe. This is roughly the number the United States will have deployed in Western Europe by the end of this year. In order to come down to this number, the Soviets would have to make a massive cut in the 243 SS20 missiles that it said it has deployed within range of Western Europe. The United States also proposes that the Soviet Union make a proportionate cut -- nearly half -- in its 170 SS20 missiles deployed in Asia.