The Democratic National Committee stands poised to perform a feat this weekend it hasn't managed in nearly two decades: approve a set of presidential nomination rules basically unchanged from those of the previous election.

But a funny thing is happening to the party on its way to stability. Southern Democrats have a scheme that could push the nomination contest into uncharted waters -- and do so entirely within the rules.

Their proposal would create a southern regional primary on March 8, 1988. It doesn't need DNC approval (states are free to fix their own dates on the primary calendar, within an overall DNC "window"), and so it won't be among the recommendations the party's rules-making body will submit this weekend to the full DNC. But it's sure to dominate the hallway and caucus chatter here.

One state, Kentucky, has changed its presidential primary to March 8 -- the date on which the Florida, Alabama and Georgia primaries already were scheduled. In Virginia, Gov. Gerald Baliles (D) is expected to a sign a similar bill, and bills have already passed one chamber in five other state legislatures -- Maryland, Mississippi, Missouri, Oklahoma and Tennessee.

The prospect of so many state contests so soon after the Iowa caucus and New Hampshire primary has led critics to worry that the DNC is sleepwalking into a de facto national primary. In 1988, two-thirds of the summer's convention delegates might be elected or committed by mid-April.

Just what impact that will have, no one is sure. The conventional wisdom -- that "frontloading" helps front-runners -- was exploded in 1984 by Sen. Gary Hart (D-Colo.), whose bid got a big boost from his surprise New Hampshire win.

Whatever the impact, the South seems determined to give the idea a try. James Van Hecke, the new Democratic chairman in North Carolina, said he spent two weeks informally polling his state and found that "everybody likes it."

Robert Slagle, the Democratic chairman in Texas, said, "Texas is damn tired of Iowa and New Hampshire exercising a disproportionate impact on the outcome." He said the chances were "excellent" that his state legislature would join the parade next year.

DNC Chairman Paul G. Kirk Jr. said, "I don't think we're going to wind up with as many states moving as people are saying now."

Some critics contend that, in the name of boosting a centrist candidate, the South may improve the outlook for the liberal Rev. Jesse L. Jackson, who in 1984 ran best in the South.

One of Jackson's advisers, Ronald Walters, has called that kind of argument "racist," and Jackson himself professes delight at the prospect of a regional primary. "The South shall rise again," he proclaimed yesterday.

Jackson, however, had a different beef about the rules. He accused the party of engaging in "Marcosism" for refusing to eliminate the threshold of primary or caucus votes a candidate must attain in order to be awarded delegates. The rule-makers propose lowering the 1988 threshold from 20 percent to 15 percent.

On a another matter, Jackson announced that his National Rainbow Coalition would hold an April 17-19 convention here to formulate a plaform and endorse candidates in this year's elections.