The MX missile may not be a bargaining chip with the Soviets in Geneva. But it sure is with Congress in Washington.

The commander-in-chief is dickering like a used-car salesman trying to unload an Edsel on a customer just in from the country. "Sure, maybe the basic product is a heap, but look what you get with it," he is telling them. "We'll throw in a stereo, air conditioning and a bar in the back."

The bottom line, not quite spoken, is pass the MX program and don't come back here looking for arms control.

President Reagan's preposterous pitch is that the only way to be sure of arms reduction is first to build the biggest, deadliest nuclear weapon on the drawing board. Buying the MX will not only prove to the Soviets that we are "serious" about defense--the possession of 10,000 nuclear warheads has failed to convince them--it will also give the peacemaker in the White House an apparently long-sought opportunity to show that he is "serious" about disarmament.

What you get with an MX basing mode that even the missile's most ardent advocates were laughing into the ground a year ago is a host of extras that you can't get any other way.

You get, for instance, a letter from the president in which he says unequivocally, "Honest, I'm for arms control."

You get the promise of a new negotiating position at Geneva, which could even take the form of the "Mutual Guaranteed Builddown of Nuclear Forces," the title of the brainchild of Sen. Sam Nunn (D-Ga.) and Sen. William S. Cohen (R-Maine), which has been endorsed by 45 members of the Senate.

The scheme provides for the retirement of two old weapons for each new one deployed. It is a shelter for those who are chilled by the nuclear freeze recently voted in the House. Its potential for arms control can be gauged by the fact that that famous dove, Defense Secretary Caspar W. Weinberger, enthusiastic about it, said, "That's what we are trying to do."

Its most appealing aspect is that it gives almost half the Senate an alibi for voting against the freeze, that inconvenient movement that the public understands and supports.

There are more fine things in the fine print of the contract that Reagan is waving at the droves of members of Congress he is receiving in relays at the White House.

You get, at a cost of billions, the wonderful new "Midgetman" missile, a single-warhead weapon that is truly mobile and could, in fact, be in the truck in front of you on the 14th Street Bridge at rush hour. You will get a thousand of them if you play your cards right.

You may also get a special commission on disarmament, a permanent group, which would, in the words of its principal promoter--again Nunn--"establish the kind of bipartisan consensus that existed during the 1960s." If it runs to form, it will recruit hawks from previous administrations and issue reports that will provide "continuity," although not necessarily arms control.

But the pressure from the White House, the visits, the hand-delivered letters are working.

The House Appropriations defense subcommittee voted 9-to-3 for the new basing mode, the Minuteman III sites, of which Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee said in 1981: "By stuffing the MXs into fixed silos, we're creating just so many more sitting ducks for the Russians to shoot at."

What moved the members baffles subcommittee Chairman Joseph P. Addabbo, the genial New York Democrat who presided over the 65-vote margin of defeat over the "dense pack" basing mode last year.

"They gave me no logic," says Addabo, "but all kinds of reasons. They're talking about 'Midgetmen' and moving the president toward arms control."

Sen. Ernest F. (Fritz) Hollings (D-S.C.), a presidential candidate and foe of the MX, is hopeful that members of the Senate Appropriations Committee will strike a blow for common sense.

"They talk about MX as an expression of political will. I was at Helsinki during SALT I, and the Soviets know what we need for defense. They know our weaponry. They aren't taken in. MX is no bargaining chip for them."

Sen. Mark Andrews (R-N.D.), a conservative, observed at a Senate Appropriations Committee hearing Tuesday that we are simply giving the Soviets "more bang for their buck" by putting the MX, with three times the number of warheads of previous occupants, into already targeted holes.

Yesterday he was summoned to the White House for an intimate talk with the president, the vice president and White House chief of staff James Baker III.

"I told the president I didn't think he needed the MX to go to the table," Andrews said.

You would think the president might be embarrassed to be saying, in effect, that without the MX he was only kidding about arms control up to now. But with customers like Congress, which is dying to buy a bill of goods, there is no need to be self-conscious.