THE OLD DREAM of southern populists -- a coalition of blacks and poor whites -- has been a will o' the wisp for most of southern history. For years most southern blacks were denied the right to vote; then, when they could vote, for several years almost all whites voted reflexively against the candidates blacks favored. Now the vision of a biracial populist coalition is being invoked again, this time by the national Democratic Party. Only in the event that such a coalition can be assembled does the Mondale-Ferraro ticket have a chance of winning electoral votes this year in the South.

The signs are not auspicious. It's something of a landmark when 14 southern Democratic state chairmen meet with Jesse Jackson and representatives of the national party, as they did here last week. But Mr. Jackson, for reasons which cannot be explained away as simple racial prejudice, is unpopular with southern white voters. A recent poll by the Joint Center for Political Studies shows that a Jackson endorsement makes white southerners less rather than more likely to support the Mondale-Ferraro ticket by a 19 to 8 percent margin.

The Democrats' strategy is to turn this around by, in Mr. Jackson's words, "an economic agenda that can meet human needs." The same idea, in different words, was the hope of the populist strategists of the 1890s. They, like Mr. Jackson, pointed out that most southern whites were clustered near the lower end of the national income scale. But American voting habits have never been a simple function of economic status. It was when southern whites had incomes far below the national average that they balked at voting for candidates supported by blacks. Only in the 1970s, when southern income levels approached and in some cases exceeded national averages, they were more willing to do so.

And of course economic issues are not the only basis for voters' decisions. It's true that Jimmy Carter's winning coalitions in Georgia in 1970 and in the South in 1976 resembled the blacks-plus- (relatively)-poor-white model. But he lost many of those votes in 1980. White southern voters tend to support an assertive, even aggressive foreign policy and to favor increased defense spending; Jesse Jackson's praise of Fidel Castro, and Walter Mondale's emphasis on arms control, are not vote- winners in Dixie. White southern voters also tend to oppose the Democrats' positions on many cultural issues.

The Democrats, by daring to feature Jesse Jackson in their southern effort, are gambling that by inspiring a large black turnout and emphasizing economic issues, they can overcome the deficit they face on foreign and cultural issues. It's a long shot.