IN THE 1968 PRESIDENTIAL primaries, candidate Gene McCarthy used to jibe that his fellow-Democrats had sold out on Vietnam for dinner invitations to the White House, which they subsequently encased in plastic and kept on the mantelpiece.

The price seems to have gone down considerably since then.

Capitol Hill looks like a gym these days as members flip, whirl, turn and do headstands to support of the MX missile, which they recently condemned as wasteful, provocative and just plain daffy.

But in persuading Congress that arms control can be achieved only through more arms, the president didn't have to serve any tender veal and raspberries out of season.

No, Ronald Reagan is pulling off his spectacular sting with comparatively little effort. A call from Air Force One, a knee-to-knee chat with Himself in the Oval Office, a hand- delivered letter here and there was enough to persuade a surprising number of statesmen that they had failed to see the beauty of the hydra- headed monster.

And the wonderful thing about it is that Reagan sends his visitors out into the everyday light in the White House driveway convinced that they have converted him.

Rep. Albert Gore (D-Tenn.), an ambitious young man who wishes to be senator, came away exulting that he and two Democratic colleagues had "really turned him around on arms control."

Happy, relieved cries of "bipartisanship" fill the air.

Another convert, Sen. Warren Rudman (R-N.H.) a hardheaded, erstwhile MX sceptic, was won over after a half hour of man-to-man in the White House.

According to his account, Reagan was "a little stunned" when the senator told him that the people of his home town, Nashua, N.H. don't think "this administration is serious enough about arms control."

If the president looked surprised at this disclosure, he must have been giving a performance of Academy Award caliber. Even his preferred periodical, the Reader's Digest, has had reference to the nuclear freeze -- albeit from the standpoint of Soviet infiltration of it. As a dedicated television-watcher, Reagan could hardly have failed to notice the peace demonstrators boiling through the squares of Europe.

Rudman denies that he was an easy mark. He says he did some hard bargaining of his own and capitulated only after he convinced the president to consider the Guaranteed Nuclear Weapons Builddown. No builddown, no MX, Rudman says he said.

The builddown, which is acquiring considerable chic on Capitol Hill among senators who like to say that the freeze is "too simplistic" -- which it may well be, since it stops the arms race by stopping ?it -- sounds like a a defense contractor's dream. It provides that for every two old nuclear weapons sent to the boneyard one new one can be built.

To doubters, the builddown looks suspiciously like a buildup of deadlier and more accurate missiles, and its sponsorship -- Sen. Sam Nunn (D- Ga.) is its originator -- gives them pause. A defense-minded Southerner, Nunn has never been associated with the cause of disarmament.

Buildown arithmetic is a bit complicated, too. If we go forward with the thousand warhead MX system, presumably we might have to put most of the Minuteman force of 2,100 warheads on Social Security.

But the other night at his press conference, the president promised that, with his born again allies, he is "exploring" the builddown, and Rudman, says, "That's worth $18 billion to me." Eighteen billion is the up- front money needed to get the MX off the ground.

Some did't need to sit in a presidential wing chair to rethink the "Peacekeeper." The pressure was self-induced.

Rep. Dan Glickman (D-Kan.) put the fear factor forthrightly: "It's not to our advantage to let him point fingers and say we killed his major weapons system."

The idea of "losing the MX" is as chilling to some Democrats as the terror of being acused of "losing" El Salvador. The president has not hesitated to do a little red baiting from time to time, and who would want to lose the arms race to the Evil Empire.

The weird thing about all this is that the country is against further nuclear buildup and involvement in Central America. But the president is going up in the polls, and it is not wise to take chances -- on the nuclear threat, perhaps, but not on November.

The MX was peddled by the Scowcroft commission as an expression of "national will." The selling of it is a dazzling demonstration of presidential will, and proof that it is easy to scare congressmen out of their wits. They'll even do it themselves.