ONE THING the first exit pollers aren't going to be able to tell you about the returns Super Tuesday is the size of the turnout. That may also turn out to be the single most important thing about the real Super Tuesday in November. The exit polls pretty reliably indicate the candidates' percentages, but they don't gauge turnout very well. For the full story, you have to wait for the last returns to trickle in and the absentees to be counted. But those results may be worth waiting for.

The reason why is that many southern states allow voters to choose candidates in either primary, and so their choice of party -- as well as candidate -- tells something about the general election. Take Alabama. In its last race for governor, 940,000 people voted in the Democratic primary -- about par for a seriously contested race. But in the 1984 presidential primary, when Walter Mondale, John Glenn, Jesse Jackson and Gary Hart were the candidates, only 428,000 Alabamians voted in the Democratic primary. In retrospect this provides a pretty fair forecast of the general election, in which the Democratic ticket lost Alabama 61-38.

Georgia, which cast 900,000 to 1 million votes in state Democratic primaries a few years ago, cast only 648,000 in the 1984 presidential primary. In Tennessee 740,000 people voted in the 1986 Democratic primary for governor, but only 322,000 voted in the 1984 Democratic presidential primary. There's been a trend for the Democratic vote to shrink in state contests as well, and for the Republican primary vote to increase. This is most apparent in Texas, where in 1978 some 1.8 million people voted in the Democratic primary for governor and 158,000 voted in the Republican primary. By 1986, Democratic primary turnout had fallen to 1.1 million and Republican primary turnout had risen to 544,000 -- and the Republicans won the general election by a solid margin.

The southern Democratic legislators who established a regionwide Super Tuesday hoped it would attract candidates with appeal to the kind of southerners -- white, culturally conservative, assertive on foreign policy -- who have been electing Democrats in state elections but have voted solidly Republican in presidential general elections. They had in mind voters like the 512,000 Alabamians who voted Democratic in the gubernatorial, but not the presidential, primary. Turning out such voters is central to Albert Gore's strategy and part of Richard Gephardt's plans as well. On the Republican side, Pat Robertson is trying to win votes from many who have voted in Democratic primaries or who haven't voted much at all in the past.

How well these politicians succeed, the turnout figures will tell. It's not clear where Mr. Robertson's new voters will go in November. But it's probably true that if the Democrats' turnout is low, they're not going to carry many southern electoral votes, no matter whom they nominate.