You know how desperate the Democrats are when they import a Republican to tell them how to play the game. At their White Sulphur Springs conference last weekend, they brought in David R. Gergen, super-flack for Richard Nixon and Ronald Reagan, to give them advice. Gergen's counsel: Keep the message simple.

But keeping the message simple is not in the cards. The big story that came out of the polished marble and high tea of the Greenbrier resort was another embarrassing instance of how hard it is to send a simple message out of a divided party.

Specifically, what emerged was a report that liberal Democrats are worried stiff that Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, is about to welsh on what they thought was a promise from him to vote with the House Democratic Caucus against the MX missile when it comes up this month.

Aspin, whose reversal on the MX in 1982 was a critical factor in its survival, gave assurances to liberals that he would be with them next time. Now, however, he is refusing to commit himself.

Rep. Howard E. Wolpe (D-Mich.), one of those who was under the impression that Aspin had seen the light, confronted him at White Sulphur Springs, W.Va., about his seeming change of heart. Two days before, Rep. Tony Coelho (D-Calif.), chairman of the Congressional Campaign Committee, had brought up the matter with Aspin.

The administration is putting on a great drive for the MX, which is the most expensive and deadliest weapon system yet devised. The argument, made with a straight face, is that a vote for the MX is a vote for peace and that opposition to the MX will somehow sabotage the upcoming Geneva arms talks with the Soviets.

Democrats fear Reagan on all counts, but nowhere more acutely than on national security matters. They know he will accuse them of sending him to Geneva with one hand tied behind his back, of displaying, once again, "weakness on defense." They began circulating rumors at their luxurious retreat that Aspin's new coyness was caused by a sudden faintness on the part of Speaker Thomas P. (Tip) O'Neill (D-Mass.), who did not attend the healing seance. The speaker, it was whispered on the tennis courts, was reconsidering his MX opposition.

O'Neill's counsel, Kirk O'Donnell, says that is "absolutely untrue." What the speaker has said was that he wants the Senate to vote first, so that the country will see that opposition to the monster is bipartisan. Chances of downing the MX in the Senate, which has tied twice on the issue, are brighter now, although not exactly brilliant. Its foes say their margin of victory is one.

The speaker has said Aspin is the key to the outcome in the House.

Aspin was once one of MX's severest critics, trashing it as useless, destabilizing and vulnerable. But in 1982, he was seduced by the Scowcroft Commission. The panel called the weapon a lot of bad names but concluded that it was essential to arms negotiations and as an expression of the "national will." Aspin told the House he was striking a hard bargain with the White House in exchange for his support, a claim that inspired Rep. Les AuCoin (D-Wash.) to observe that he was glad that Aspin was not negotiating with the Soviets.

When, late last year, Aspin began running for the chairmanship of the Armed Services Committee, he assured about half a dozen liberal colleagues -- so they thought -- that he would vote with them against the mammoth symbol of the arms buildup.

But the witnesses he called before his defense policy subcommittee aroused suspicion: SAC commander Gen. B.L. Davis, Brent Scowcroft of the Scowcroft Commission, former defense secretaries James R. Schlesinger and Harold Brown -- friends of the MX. The final witnesses will be powerful fans, Secretary of Defense Caspar W. Weinberger and Secretary of State George P. Shultz.

Some members want a delay so that the MX can be decoupled from the arms talks. Many Democrats hate having the first big vote of the season be another nay against a nuclear weapon, although Coelho and others suggest there is no better way of showing the difference between them and the Republicans.

But the White Sulphur Springs conference, which was financed by big corporations and which admitted lobbyists but not reporters to its business sessions, was put on by Democrats who think that they have to be more like Republicans. It follows the formation last week of a conservative Democratic Leadership Council, which political consultant Robert Squiers calls "a southern male caucus."

Some Democrats would like to define themselves by defying Reagan on questions like the MX. But me-tooism, as the importation of Republican therapists proclaims, is the impulse of the day.