Two key congressional figures have taken the unusual step of urging the Reagan administration in writing to drop its idea of deploying the new MX missile in airplanes, on grounds that it's a bad idea that would be "virtually impossible" to get Congress to approve.

This bluntest warning yet against the air-mobile MX option came from Chairman Melvin Price (D-Ill.) and ranking Republican William L. Dickinson (Ala.) of the House Armed Services Committee, which must authorize the billions needed for the new missile.

Their objections, spelled out in a letter sent last week to Defense Secretary Caspar W. Weinberger, come atop similar opposition from Chairman John Tower (R-Tex.) of the Senate Armed Services Committee.

This confronts President Reagan and Weinberger, who are to talk about the MX at the president's ranch next week, with a potentially embarrassing political problem. Reagan assailed Jimmy Carter in last year's presidential campaign for delaying deployment of the MX. Now Reagan, too, could end up having to go back to Square One. There seems to be no quick and easy way out of the MX deployment issue, on which all sides are now lobbying furiously within the administration and on Capitol Hill.

Reagan, who attacked Carter's MX deployment plan during the campaign, could hardly now embrace that idea, whilch was to hide 200 MX missiles in a giant subway system in Nevada and Utah. But the Air Force fallback proposal of starting out small by putting just a few MXs on military property in the West would not do much toward closing "the window of vulnerability" the administration has deplored; there is no Air Force general in sight willing to help Reagan overcome the growing opposition to basing the MX in the sky, and it would also take enormous effort to sell Congress on other previously rejected ideas, like putting the MX to sea on ships or in submarines or stuffing existing Minuteman silos with the new missile.

The Price-Dickinson letter, written on the basis of information supplied by the Pentagon itself, dramatizes the depth of arguments Reagan and Weinberger would have to overcome if they clung to the air-mobile option, which Weinberger is now reported to favor. Here are highlights of that letter:

Presidents Ford, Carter and the Defense Systems Acquisition Review Council -- after in-depth analyses-- all rejected the air-mobile option because it would be costlier and leave the missiles more vulnerable than the Air Force scheme to move MX missiles from one ground shelter to another to make them hard to hit. Defenders of the new air-mobile plan claim that it overcomes the past objections on cost and vulnerability by using fuel-efficient airplanes that can stay aloft for long periods.

"Only 40 percent of an alert force of 34 C5 aircraft each carrying one MX missile could survive a Soviet coordinated attack" by land and submarine missiles, according to the Pentagon's own studies. "Just a few minutes of delay in reaction time would reduce the number of survivable RVs re-entry vehicles which carry the MX warheads to zero."

"It would not really maintain a triad" of land-, sea- and air-based nuclear weapons if the land missile leg represented by the MX "is deployed in the same mode as the bomber leg."

The air-mobile MX would cost almost twice as much as the land version -- $800 million versus $440 million a year to build, operate and maintain.

The Soviets would have to target only five H-bombs to stop the 10 warheads on an air-mobile MX, an "unfavorable" ratio compared to the "favorable" one offered by land deployment. The latter would force the Soviets to commit two warheads to each of the 4,600 shelters to be sure of getting the 200 missiles moving among them.

Airborne gadgetry for aiming and firing the MX is "relatively soft and susceptible to jamming."

Even if air-mobile MXs survived enemy attempts to stop them before launch, "the accuracy of the air-mobile system in the 1990s is expected to be only as good as that of the current Minuteman force" of land missiles.

"Environmental requirements for an air-mobile system could be more stringent" than those which have arisen in the land-based plan advocated by the Air Force. Price and Dickinson fear there would be lawsuits to try to prevent the military from flying MX missiles for as long as two weeks at a time and from buying land for widely scattered landing areas.

"I can tell you right now," said one veteran congressional staff member with a sure feel for the politics of defense, "that air-mobile MX is dead up here. Congress is not about to appropriate billions for something the military tells them is a bad idea. The Pentagon is not HEW," the former Health, Education and Welfare Department that Weinberger used to head, "where everybody falls in line when the boss says, 'This is it'."