Illinois could be the turning point for the Democratic presidential campaign in 1984. If Walter Mondale captures the Illinois primary tomorrow, his comeback bid against Gary Hart will be right on target. It would give Mr. Mondale the aura of a winner, as he fights his way toward the New York and Pennsylvania primaries in the next few weeks.
But if Senator Hart triumphs in Illinois, Mr. Mondale's prospects could take a steep dive. He would have to begin rebuilding confidence in his campaign - with time and money fast running out.
The Tuesday vote will test for the first time whether the Mondale coalition of blacks, big labor, and hard-core Democrats can stop Hart in a Northern industrial state primary.
Hart is doing his best to split that coalition. His major appeal is directed at independents and ''soft'' Democrats. But he is also trying to take advantage of fissures in the ranks of Chicago Democrats. The party in Chicago has been divided along black-white lines since the election last year of Mayor Harold Washington.
Illinois is the last of three hurdles that Mondale needed to cross after his sudden, unexpected upset in New Hampshire.
Hurdle No. 1 was ''Super Tuesday,'' March 13. Mondale had to win in at least two Southern primaries to slow Hart's momentum. Mondale did it - but just barely.
Hurdle No. 2 was ''Super Saturday,'' March 17. It was a ''must win'' situation again for Mondale in the Michigan caucuses, where he was strongly backed by the United Auto Workers. As expected, he trounced Hart with about 50 percent of the vote. Mondale also led in the Arkansas and Mississippi caucuses that same day.
Hurdle No. 3, the Illinois primary, could be the stiffest test of all in Mondale's comeback effort, however.
Several aspects of the Illinois contest suggest that the political currents will be especially tricky for Mondale on Tuesday.
Jesse Jackson, who got strong black support in several Southern caucuses and primaries during the past week, has the backing of Chicago Mayor Washington. Mr. Jackson is expected to draw away tens of thousands of blacks who would otherwise go into the Mondale column.
This puts Jackson in the role of a ''spoiler'' in Illinois - drawing votes away from Mondale, and tilting the election toward Hart.
Jackson played that same kind of ''spoiler'' role last week in the South. His heavy black support almost lost Georgia for Mondale, and it seriously undercut Mondale's vote in Florida, where Hart came out on top.
Another complicating factor for Mondale is the withdrawal of John Glenn and George McGovern. Many of their supporters are expected to gravitate toward the young senator from Colorado, rather than Mondale. Voter surveys have indicated that a large portion of the Glenn and McGovern strength came from Democrats and independents whose views can be summed up as ''anybody but Mondale.''
Polls in Illinois show the Mondale-Hart race too close to call.
While there is a great need for Mondale to win, Hart also must keep proving himself. If nothing else, Hart must dispel the oft-heard opinion that he could be just a ''media phenomenon'' - a two-week wonder with whom the voting public quickly loses interest.
Among other things, Hart strategists want to disprove surveys showing that in recent days, more voters have been switching to Mondale than Hart. Newspaper stories and TV reports critical of Hart have escalated in recent days, and Hart aides are concerned that this is already slowing the stride of his campaign.
Some of Hart's latest media problems have been brought about by miscues among his own, inexperienced staff. Over the weekend, for example, the Hart campaign withdrew a television advertisement that linked Mondale to ''bossism.'' The ad attacked Cook County (Chicago) Democratic chairman Edward R. Vrdolyak, who supports Mondale in the primary, and who has been battling with Mayor Washington.
Hart's team hoped the ad would build support for their man among blacks and independents. But the ad was scrapped after it was decided that such a personal attack would be a tactical blunder. Hart is seeking to hold the campaign debate to issues, rather than open himself up to sparring over such questions as his age.
If there was one bit of bad news for Mondale, it was Jackson's better-than-expected showing in some of the caucuses on Super Saturday.
Jackson, in his native state of South Carolina, emerged with 25 percent of the vote, more than Mondale and Hart combined. Uncommitted delegates won about 50 percent of the vote. Mondale's showing of only 10 percent prevents him from getting any delegates there. The South Carolina showing is certain to encourage Jackson to carry on his effort.
Jackson also did well in Mississippi, where incomplete returns indicate he finished second, just behind Mondale, with more than twice as many votes as Hart.
Both Mondale and Hart cheered the Super Saturday results. Hart claimed that his showing in Michigan, with about 40 percent of the votes, was excellent in light of Mondale's union allies there. Michigan is the nation's second-most heavily unionized state, after New York.
Mondale beamed as the results came in. ''We did well on Super Tuesday,'' he said. ''We did better today. We're fighting back.''