Walter Mondale, having had three months to think about it, now thinks he knows why he was crushed in the Reagan avalanche last November.

It was, he told a Washington's Birthday gathering of the leaders of the AFL-CIO, because of his "failings" as a TV performer.

Mondale's was but the latest in a series of post- mortems of last year's debacle. Jesse Jackson is persuaded the party's big mistake was in its mindlessly macho move toward white males: a minority of the electorate. And any number of no-charge analysts has offered that the Democrats are in trouble because they allowed themselves to be branded as the party of special interests. It isn't so much that the analyses are wrong as that they are irrelevant.

Mondale is surely correct in his judgment that he is not a compelling figure on television, correct also in his prediction that, in future elections, candidates who "don't know how to use that tool, and use it exceedingly well, are going to be at a disadvantage."

But anybody who watched last fall's presidential debates will remember that President Reagan, for all his legendary reputation as actor, communicator and master of the tube, didn't do very well on television. Indeed, it was the general judgment that Mondale won the first debate, and pretty much held his own in the second. Greater mastery of the medium clearly would have been to Mondale's advantage, but it was not television that cost him the landslide.

It is also true, as Jackson claims, that Mondale's campaign soft-pedaled the appeal to black voters (obviously fearing the deadly pro-black label) and sought to make inroads into Reagan's white-male preserve. But Jackson's own efforts to increase black registration were essentially a wash, because conservative Republicans played off that effort to increase white registration.

And finally it is true that Mondale was seen as the creature of special interests, which he collected with the zeal of a squirrel preparing for winter, while Reagan, the darling of a more powerful set of special interests, managed to avoid such labeling. But again, that is not what cost Mondale his humiliating defeat.

Indeed, if Mondale had succeeded in capturing the special interests he thought he had, the election might have turned out quite differently. The fact is, Mondale collected not the special interests but their putative leaders: not workers but the AFL-CIO leadership; not women but NOW; not Hispanics but the leaders of the Mexican American Legal Defense and Education Fund. The only special interest group that went for Mondale was the group that he promised the least: blacks, who would have voted for almost any Democrat who challenged their archenemy Reagan.

What cost Mondale was the perception that Reagan, while perhaps on the wrong side of a number of specific issues, generally stood for what most Americans stood for, while Mondale, though often on the right side of the specifics, didn't really stand for anything at all.

What is true of Mondale is also true of the Democratic Party, which no longer seems interested in a set of unifying principles but only in a crazy-quilt of unconnected interests. Without a unifying vision, the collection of interests becomes not an electoral majority but a paranoid gang of competitors.

It is not what AFL-CIO President Lane Kirkland called Reagan's "clever management by adroit marketers" that has the Democrats in such disarray, but rather the fact that the Democrats are no longer sure who they are and what it is they wish to market.