Labor puts its endorsement on the line in Illinois

At the sprawling Chrysler plant here, two shifts of workers turn out compact cars for 18 hours a day. On most Saturdays the assembly line keeps right on going. Even so, Chrysler can't keep up with all the orders for the Omni, Horizon , and Shelby cars it makes here.

This is prosperity. Paychecks are fat, and they come in steadily. After a 54 -hour workweek, employees are glad just to get a day of rest.

Despite the good times, however, many of the union members here have not forgotten that their jobs were on the line a few years ago. Chrysler was near bankruptcy. Some politicians in Washington wanted to let the company fold. But a number of others, including Walter Mondale, came to their rescue.

''Walter Mondale opened his house in Washington for union members,'' recalls Don Margraf, a United Automobile Workers official here. The union and the Chrysler Corporation lobbied Congress and eventually won federal help that saved the company - and more than 4,000 jobs here in Belvidere.

''Gary Hart says Chrysler should have the right to go under,'' says Mr. Margraf. ''Well, 4,000 people would have been out of work, and maybe 10,000 more in related jobs in this area alone.''

Not every union voter, however, has seemed so grateful to Mr. Mondale this year. The former vice-president has won the solid support of union leaders. But in states like New Hampshire and Massachusetts, the union rank and file mostly went with Senator Hart.

This kind of result has befuddled political analysts.

When Big Labor first wrapped its muscular arms around Mondale last fall and pledged its support, just about everyone thought that assured him the Democratic nomination.

It hasn't worked out that way. Some have even suggested that labor's endorsement has hurt Mondale with other, nonunion, voters.

Such judgments, however, probably should not be made too quickly. The pivotal test for Big Labor is now under way. In Michigan, auto workers helped Mondale trounce Hart in the caucuses last Saturday. Here in Illinois (where they are counting primary votes as this story goes to press), labor is expected to help Mondale. And in even bigger tests ahead - New York and Pennsylvania - labor could be the difference between Mondale and Hart.

Whatever the outcome in Illinois, labor claims the tide of this Democratic campaign is already swinging its way, and therefore Mondale's way.

Says an AFL-CIO coordinator in Chicago: ''If (the Rev.) Jesse Jackson weren't in this race (drawing away black votes from Mondale), we would clobber Hart.''

Still, the large number of unionists who back Hart troubles some AFL-CIO officials. They suggest there are two reasons for it.

First of all, younger union members (just like other young voters) seem to be the ones principally drawn to Hart. Says the UAW's Mr. Margraf:

''Some of these people are new to the union. They've had it pretty good. They don't really know the issues. They've never been laid off. They've never had their plant close. They can't relate to what anyone has done for them. And they like the image of Hart. He's in fashion.''

Voter studies seem to bear that out. Older union members are overwhelmingly for Mondale, younger ones often lean to Hart.

There's another factor. Hart has done far better with union members in states like New Hampshire where unemployment is low. But where unemployment is in double digits, such as Alabama, Mondale has whipped Hart among union voters by nearly 3 to 1.

Recent economic hard times all across the industrial belt should work in Mondale's favor in coming weeks.

It was those hard times, in fact, which helped turn union leaders toward Mondale in the first place. They felt he would do more for average American workers if the going got tough.

Ben Albert, an AFL-CIO political specialist in Washington, has detailed several key differences between the two front-runners. Among them:

* Jobs. Mondale favors, Hart opposes a bill to require that cars sold in the United States contain a certain level of American-made parts. Unions charge that Hart is for free trade at a time when other nations are guilty of unfair competition.

* Youth. Mondale opposes, Hart favors a subminimum wage for young people. Unions charge such a law would have a revolving-door effect - pushing adults out of jobs while bringing youths in.

Union officials concede Hart isn't all bad. His votes in Congress are ''right'' about 79 percent of the time, Mr. Albert notes. But Mondale got a 93 percent rating.