LEGISLATORS IN half a dozen southern states will be making decisions in the next few weeks that could result in one thing almost everyone who has thought about the subject for more than five minutes doesn't want: a national presidential primary. That's not their intention, of course; but if they make the wrong decisions, they may not get the southern regional primary they want either.

On the surface the question is whether five to nine states will join the four that already have scheduled primaries for the week of March 8, 1988, one week after New Hampshire (and Virginia, where a bill awaits Gov. Baliles's signature). The real question is not whether a lot of southerners will vote on March 8 (they did in 1984 and will in 1988), but whether southerners want to risk everything on a single day's primaries rather than set up a series of southern tests -- a southern regional gauntlet. Decisions will come soon. In five states -- Maryland, Mississippi, Missouri, Oklahoma, Tennessee -- one house of the legislature has voted a March 8 date. They could amend their bills and set a March 15 date. Southern states acting later -- Arkansas, Louisiana, Texas and the Carolinas are interested -- could schedule their contests for the week of March 22.

The originators of the southern regional primary proposal feel this would dilute southern influence. But you can argue that, in fact, it would increase it. Voting all at once at the earliest possible date, the South could easily find itself in a position of ratifying the choice of Iowa and New Hampshire. Voting in three successive weeks, it could hold the national spotlight three times as long and just as effectively. It may be objected that it will have to share the spotlight. But it will in any case. Massachusetts, Rhode Island and Vermont are already scheduled to choose their delegates March 8, and there is noth states from joining them. There is talk of New Jersey, the Rocky Mountain states and California all moving to the early date, and Illinois could schedule its March 15 primary one week earlier.

There is no way then the South can monopolize March 8, and if it tries it risks turning that early date into a de facto national primary. In that case you won't see any more candidates and network anchors prowling the streets of Nashville or Biloxi than you do now, and whatever regional values and goals the South wants to promote will be lost in a coast-to- coast struggle for national air time. And of course the nation risks getting one or two nominees, and maybe a president, that it has not gotten a chance to know thoroughly and who may not hold up under scrutiny over time.

There is something to the argument that increasing southern influence would produce more representative national tickets. But a better way to produce nominees who can win would be to set up a mostly southern gauntlet the three middle Tuesdays in March rather than staking everything on what could easily become a single national primary.