Illinois primary - a pivotal testing time for Hart and Mondale

All of a sudden, Illinois is center stage. Both front-runners in the Democratic presidential campaign are here, battling for attention. In Chicago, Gary Hart answers the foreign-policy criticisms of his chief rival. Downstate, Walter Mondale visits farmers and calls the coming Illinois primary ''a shootout in the OK Corral.''

Even the local news media are echoing the message, with the Chicago Tribune labeling the state as ''the next New Hampshire.''

What is going on here?

According to political pundits, next Tuesday's primary here may well mark a turning point in the Democratic campaign. This is probably Mr. Hart's first real chance to demonstrate whether he appeals to Midwestern voters. And, in the words of one observer, ''Mondale's best chance to slow down the Hart bandwagon.''

The state is also a very tough state to call - so diverse that it is easier to tell how it will split apart than how it will come together next Tuesday. The race between Hart and Mondale is very tight, observers say.

Politically, Illinois is an odd mix. Its south is conservative with a historically Southern flavor; its midsection is wide, and sturdily Republican; farther north it is uneven, with patches of Republicans and two swing counties. And then there is the solidly Democratic city of Chicago.

But there are other ways to slice the pie.

Labor is strong, with 1 million union members in a rich mix of industrial, building-trades, and public-sector unions. Its credibility to produce votes for Mr. Mondale is on the line here, observers say, after mixed performances in earlier contests.

But, says John Jackson, a political science professor at Southern Illinois University, ''it's also true (that) labor's not able to do it alone.''

Mondale's strength among blacks and even the traditional Democratic Party organization will also be tested, observers say.

Blacks, who make up 40 percent of the Chicago electorate, could easily flow to Jesse Jackson, who, though not born here, enjoys the status of favorite son in the city. ''I think Jackson will be a real spoiler for Mondale,'' says David Greenstone, a political science professor at the University of Chicago.

The city's political machine, less powerful than under former mayors, is pushing for Mondale. But here too the machine is much more concerned with its own local races than presidential politics.

''It's always been that way,'' one close observer says. ''There's no great excitement among the party regulars to get out the vote for Mondale.''

One other significant split: The Chicago-downstate rivalry runs deep. Even in high school basketball, for example, downstate coaches cheer if a non-Chicago area team wins the state championship. In politics, it means a perverse kind of voting sentiment, an observer says. If the machine backs Mondale, downstaters are more likely to vote for Hart.

Since the presidential race in the Democratic primary appears much more exciting than many of the Republican races, observers expect some Republican crossover votes to Hart. He may also pick up votes from supporters of John Glenn.

It shouldn't come as any surprise that the Illinois primary itself is split in two.

Mondale is almost certain to win the delegate count, since Hart does not have full slates of delegates running in all 22 of the state's congressional districts. Hart is concentrating instead on the equally important, but nonbinding preference vote, a kind of ''beauty contest'' that could provide crucial momentum for the important New York primary April 3.

In the most important statewide race, Sen. Charles H. Percy (R) is considered an easy favorite in the primary against US Rep. Tom Corcoran. But the two-time incumbent could face a strong challenge in November, observers say, particularly if Rep. Paul Simon wins the Democratic primary.

Latest polls put Mr. Simon, considered the front-runner, ahead of the three other candidates. But figures showed attorney Alex Seith catching up.

It is essentially a race of Simon, the downstater, vs. Mr. Seith, from the Chicago area.

In 1978, Seith was the Democratic challenger to Senator Percy. And though Percy won by a confortable margin, preelection polls showed Seith virtually in a dead heat.

Statistically, says Professor Jackson of Southern Illinois University, if a senator is reelected his first time, the next toughest race is for the third term, which Percy is trying for.