LITTLE ROCK, ARK., AUG. 17 -- The southern statehouse legislators who created the first regional presidential primary in the nation's history gathered here today both to celebrate and to fret over their handiwork.

"It is already a success," former Virginia governor Charles S. Robb (D) told nearly 1,000 members of the Southern Legislative Conference (SLC) who together turned next March 8 into a sweepstakes that will elect one-third of the delegates to each party's presidential nominating convention. Primaries or caucuses will be held on that date in 20 states -- 14 of them in the South.

"You have changed the dynamics of the nominating process," Robb continued -- but his upbeat assessment did not jibe with fears expressed by rank-and-file Democratic legislators that the calender change in itself may not help to reverse the party's 20-year slide among white voters of their region.

"The ones we have now are going to be hard to sell," said South Carolina State Rep. J.H. Nesbitt (D) of the current Democratic field of candidates. "Their thinking is not like the thinking of the people in our state."

"I don't see a candidate out there that will generate much enthusiasm," said A.L. Philpott, speaker of the Virginia House of Delegates. "It sure would help if we had a candidate of our own."

Philpott and Nesbitt are both Democrats, as are 77 percent of all state legislators in the South. The regional primary was almost exclusively a Democratic idea -- designed to yield a nominee who would have appeal in a region that remains slightly more conservative than the nation as a whole.

Many legislators here expressed disappointment that Robb chose not to run, and their continuing uncertainty over whether Sen. Sam Nunn (D-Ga.) will get nto the race -- he's expected to announce a decision within the next few weeks -- has left them, at least for now, without a prominent Southern moderate-conservative.

Sen. Albert Gore Jr. (Tenn.) is the only southerner currently in the Democratic race, but his youth and lack of prominence have made it difficult for him to line up much political support outside his home state.

If the Democrats here are ambivalent about their creation, the Republicans seem delighted by the opportunities it presents.

"We will cast this {the March 8 contests} as a liberal-versus-conservative primary," William D. McInturff, a consultant to the Republican National Committee, said, outlining a strategy to draw white conservatives into the Republican primaries on Super Tuesday.

Eight of the southern states will allow voters to choose between the Democratic or Republican primaries, and GOP strategists hope that conservative Democrats who cross over in March will stay in the GOP column the following November and beyond.

McInturff argued that Democratic primaries over the past decade have already become dominated by liberals -- "teacher activists, labor activists and blacks."

Glen Browder, Alabama secretary of state, argued along parallel lines that he anticipates low turnout in the Alabama primary, which will work to the "advantage of Jesse Jackson and to the Republican Party."

The rationale for the regional primary -- which will be held a month after the Iowa caucuses and three weeks after the New Hampshire primary -- was partly to reduce the importance of those two opening-round events. Robb characterized the Democratic electorate in those states as "small and predictable and subject to a fair degree of manipulation."

But in this preliminary year to the 1988 campaign, presidential hopefuls in both parties have broken all records for investment of time and resources in Iowa, with some having already campaigned there more than 100 days and hired paid staffs of 15 to 20 people. The happiest face Robb could put on that development was to note that the candidates also are increasing their traffic through the South in this precampaign year, and that several televised debates will be held in the region early next year that will force them to pitch their remarks to an audience larger than the "avant garde" special-interest groups in the two opening states.

Still, while many southern leaders welcomed the increased traffic, most were skeptical that the candidates were shaping their appeals to the swing southern conservative Democratic voters who went overwhelmingly for President Reagan in 1984.

And although the candidates are making more visits to the South, they typically do not linger for the sort of retail politics so common in Iowa and New Hampshire. "They tend to touch down at the Jackson airport, hold a press conference and then move on," said Rep. Cecil Simmons, speaker pro tem of the Mississippi House.

"It's going to be awfully hard for any of us to retail your states," said former Nevada senator Paul Laxalt (R), the first of eight presidential candidates to address the SLC over the next three days. "You've got to do well in Iowa and New Hampshire and hope that the sort of media hype you get out of that will create a political tidal wave for you in the South."

Laxalt, who has been running as Reagan's close friend and ideological heir, was asked how his presidency would be different. He said he would take a more activist posture toward protecting domestic industries, especially textiles, from unfair trade competition, and said he would provide tax incentives for oil producers. He added that he would bring a more hands-on management style to the White House.

Illinois Sen. Paul Simon (D), the other presidential hopeful who spoke today, pitched his southern roots -- "my home in deep southern Illinois is 80 miles closer to Little Rock than it is to Chicago," he said -- and offered a blend of populism and fiscal conservatism. He talked of his support for a balanced budget amendment, but also of his advocacy of education, health and jobs programs to address the growing disparity between the nation's wealthiest citizens and its poorest.