Democratic leaders in the South, hoping to increase their region's influence in the selection of a presidential nominee in 1988, are preparing a serious effort to change their states' primary and caucus dates so that they all fall in one week in March, most of them on the second Tuesday of that month.

The purpose would be to give the South an early -- and moderating -- influence over the selection of the Democratic nominee. By concentrating most of the southern primaries on one momentous day (Remember "Super Tuesday" in 1984? This would be "Mega- Tuesday") these leaders believe they would have a better chance to avoid having another big-spending, unelectable, knee-jerk Yankee liberal head the ticket.

And maybe they're right. But like most previous major changes or "reforms" of the Democrats' presidential selection process, this one almost certainly would trigger the Law of Unintended Consequences. "Mega-Tuesday" is an idea for those who fancy unexplored territory and daring leaps in the dark.

One unintended consequence would be to make a presidential race more difficult for newcomers and long-shot candidates. A modestly financed but reasonably diligent candidate can make a lot of hay in Iowa and New Hampshire with the expenditure of time and effort traveling from town to town, hamlet to hamlet, as Gary Hart demonstrated in 1984. His distant, 15 percent second-place finish to Mondale in Iowa propelled him to victory in New Hampshire and almost won him the nomination.

But the southerners' proposed "Mega-Tuesday" would be the next thing to a national primary. It would require lots of television, lots of jet travel and far more political manpower than in the past.

In 1984 there was a national prenomination spending ceiling of about $20 million for each candidate who took federal matching funds. The state spending limits of Iowa and New Hampshire combined were less than $1.1 million. And, as Greg Schneiders, who was a campaign strategist for Sen. John Glenn, points out, a candidate finally starts running out of things to do in those states, which frees him up for other things.

The combined spending limits for Texas, Florida, North Carolina and Georgia alone, however, come to nearly $10 million, or half the national total. There is no danger that a candidate will run out of audiences or events campaigning in a "Mega-Tuesday" regional primary.

Also, with Sen. Edward Kennedy out of the race, the chief beneficiary of a "Mega-Tuesday" could be Jesse Jackson. Kennedy was the only candidate in the current lineup who at this point could have seriously contested Jackson for the black vote in the South.

Some southerners who favor the regional primary concede there is a danger in staking everything on one roll of the dice, but they're willing to take the chance.

There is still the likelihood that the winner of New Hampshire will have the momentum to blitz many of the southern states. Hart went from 3 percent to 35 percent in Georgia practically overnight in 1984, even though he is a Yankee. The southerners might still get a liberal, and there would be no later primaries to force him to moderate his views.

On the other hand, the Democrats might wind up with candidates who can win only in the South -- like Jimmy Carter in 1980.

There is another possible consequence. The Democrats nationally have come to realize that the proliferation of primaries over the past 20 years has strengthened the influence of special interests -- the "ax-grinders," as John Sears, President Reagan's former campaign manager, calls them -- to the detriment of their candidates in the general election. A big, early regional primary magnifies this influence and, in turn, further dilutes the influence over candidate selection of party officials, who best know the candidates and can, in private, ask tough, politically pragmatic questions that the press and voter groups can't.

It's those "ax-grinders," after all, who have given the Democratic Party the presidential candidates who have driven the southerners to such despair.