Southern state legislators are on the verge of success in their drive for a southern regional primary, with most of the South voting in early March, just after Iowa and New Hampshire. It would be the biggest change in the presidential nominating system since the proliferation of primaries in 1972. But have they anticipated all the consequences? The southern regional primary is appealing at first glance, but a second look would indicate that it solves problems that are already well on their way to solution and tends to create some new ones.

The case for the southern primary is made by Texas state senator John Traeger: "We can help both parties select candidates that are truly representative." Translation: The South will prevent the Democrats from nominating a candidate too left-wing, too attentive to the caucuses of unions, teachers, feminists and gays to win a general election. By voting early and often, the South will have a veto over the nominee and an influence over the conduct of all the candidates.

But the South is already having that influence. None of the little-known and unknown Democrats likely to run in 1988 is advocating or emphasizing liberal positions on cultural issues. The one possible exception, Jesse Jackson, is not likely to be hurt by a set of southern primaries; he could run first in a split field in 1988.

More important, the South is becoming less rather than more distinct from the rest of the nation. You won't find that much difference in style or substance between northern and southern governors these days. In the House last year, southern Democrats voted almost identically with northern Democrats on the tough issues of the budget resolution, tax reform and Gramm-Rudman. The electoral mix in the South is different from that in other states, with more blacks and white fundamentalists, fewer ethnics and trendy whites. But these are differences of degree, not kind.

As William Schneider points out in the National Journal, southern Democratic primary voters in 1984 didn't differ significantly from northern Democratic primary voters on such major issues as income redistribution, Nicaragua, the nuclear freeze, gun control and affirmative action. Southern Democrats were aghast when Walter Mondale got only 37 percent of the vote in the South. But he won only 41 percent nationwide -- a four-point gap. The last two northern liberal nominees, George McGovern and Hubert Humphrey, ran nine and 12 points respectively behind their national showings in the South. The Democrats' problem in 1984 was not regional but national.

If it is not plain that a southern regional primary is needed to move the Democrats closer to a nominee who would be acceptable in the South and nationally, it is plain that it would have unpleasant consequences its backers have not anticipated.

First, it could easily boomerang and increase rather than decrease the influence of Jesse Jackson and of Iowa and New Hampshire. Jackson, so critical of so many Democratic rules, has had nothing to say against a southern regional primary. Lobbyists for Iowa or New Hampshire know that Democrats and Republicans who run well in those contests will have a huge edge over those who don't. If most of the South votes on the same Superduper Tuesday, voters there will almost certainly choose from the No. 1 and No. 2 candidates in Iowa and New Hampshire.

The other, greater danger is that the southern regional primary could easily become a national primary. That, as pollster Patrick Caddell points out, is the one way everyone agrees is a bad way to choose a president. There is nothing to stop other states from joining the move to Super Tuesday and a great deal, if it looks as if the choice will be made then, to encourage them to do so. There is talk now of the Rocky Mountain states joining in, or New Jersey, or even California. And why should they wait if it looks as if the nomination will be determined on southern Tuesday? The disadvantages of a de facto national primary are obvious. Front-runners, notably George Bush, would have a huge advantage. The voters would have little chance to scrutinize any of the candidates, to watch them in action and under stress. The chance is increased that one party or both will get stuck with a candidate who, it will soon be apparent, is just not up to the job. The choice among the thousands of politicians and public figures available -- of whom two or three or four should be a serious candidate for president in any year -- is hard to make all at once. A decentralized, serial, seemingly chaotic process to choose a single, unifying leader is appropriate in what historin Robert Wiebe describes as "a society of segments, each presuming autonomy in its own domain, each requiring homogeneity of its membership, and each demanding the right to fulfill its destiny without interference."

It is not certain that the move to a southern regional primary will occur. Its backers hoped Texas would enact one in a special legislative session this summer, but Gov. Mark White says he won't call a special session. No state has taken final action yet, and so at the moment only the four southern states that did so in 1984 are scheduled to hold primaries or caucuses early in March 1988. It is easy to understand why southern Democrats were frustrated by their choices in 1984 and are eager to increase their influence. But accident and personality -- the collapse of the Glenn candidacy, the failure of Hollings and Askew to get off the ground -- accounted for the lack of Democrats in 1984 with appeal to southern whites. The longer-term trends tend to increase the influence of the South. "The isues of interest to southerners are representative of mainstream America," says SRP advocate Traeger. If so, what need is there for a southern regional primary -- especially when it could have such dangerous consequences?