AS USUAL it's not the Republicans the Democrats need to be worrying about so much as it is some of their own exuberant, thoughtless and self-promoting minions. Democrats, the record will amply show, have a knack for doing themselves in well before the Republicans even get around to them. Consider the aftermath of Sunday night's startling debate. Candidate Mondale did unexpectedly well, candidate Reagan unexpectedly poorly. What advantages and opportunities may flow from that are already being put at risk by the reaction of some of the ticket's alleged friends.

There are first those speech-writers, coaches and consultants we have come to think of as the cuckoo- clock crowd. These are the professional advisers who can be counted on to come out with cuckoo- clock-like predictability (and a message more or less tomatch) whenever a candidate does well. It will be leaked and otherwise indirectly revealed that they (who else?) were the secret authors and architects of the success, that it was their advice (did you ever doubt it for a minute?) that was finally taken -- to great and glorious result. When things go wrong, of course, they don't come out and chirp their usual"Cuckoo!" Instead, they let it be known that they were frozen out of the advising process, that their text was the one discarded, even though it could have saved the day and so forth.

The individuals putting out this sort of information about themselves either don't care about or haven't paused to consider the implications for the candidate they are purporting to help: they make that person look foolish, empty, a puppet who is manipulated by themselves and who is incapable of whatever good thing he has just achieved. Not that this is generally the case -- but it is made to seem the case by the self- promoting consultants and speech-writers.

They have been very much on the move since last Sunday night, getting their "story out," as the recent Richard Nixon would have put it. This can only work to undermine whatever gain Mr. Mondale got from his performance. The same may be said of the mindless statements of other Democrats -- the hip-shooting Tony Coelho comes to mind -- who are now going around making sly and senseless statements about Mr. Reagan's age. It was the president's performance, nothing the Democratic candidate said or even hinted at, that seems to have brought this subject to public discussion this week. But it will be turned back against the Democrats, and with a vengeance, if they pursue it in the gross way that Mr. Coelho has tried out.

Finally, it seems to us, there is an awful lot of "Who won? We did!" gloating in the air. This, too, will rebound and possibly knock the gloaters cold. Mr. Mondale hit hard on the subject of leadership last Sunday, and evidently scored some points. If he is a leader of his own party and his own campaign he will quickly turn off the self-indulgent (and self-destructive) voices being heard so unattractively now.