In Illinois, they still like Reagan, but . . .

Illinois still belongs to Ronald Reagan. But his hold now is tentative, not the firm grip he had at the time of the 1980 election.

The judgment of observers in this populous Midwestern state is that while the recession has cut into public support for the President, he still retains the affection of the people at large, enough so that he remains formidable here as a President and as a politician.

Thus, Democrats in this state are not yet running against Mr. Reagan and his policies, at least not with any vehemence.

For example, the Democratic candidate for governor, Adlai E. Stevenson III, is aiming the full thrust of his attack on his Republican opponent, incumbent Gov. James R. Thompson. He puts the blame for the bleak economic climate here squarely on the governor. He cuffs Reagan only lightly and indirectly by calling Governor Thompson a ''cheerleader'' for the President.

One Democratic county leader, who says he thinks Reagan is ''fading fast,'' also concedes that the President ''still has the general approval of the public. He still communicates well.''

''No Democrats want to take on Reagan,'' another influential Illinois Democrat explains. ''Not yet anyway. They know that a lot of people out there still like this President a lot.''

In an interview with the Monitor, Governor Thompson revealed that a just-completed private Republican poll shows the President, although slipping in recent months, still has the majority of the state's voters behind him. Sources here add that Democratic polls also find Mr. Reagan still quite formidable.

Illinois is state whose people reflect the wide variety of voting groups all over the United States: its big, Northern industrial city of Chicago with its large, congregated ethnic groups still defying the melting-pot concept; Chicago's large suburban community, affluent and Republican; downstate Illinois, the heart of the Midwest corn belt; and southern Illinois, home of rural whites and blacks, part of the Southern Bible belt.

All is not rosy for the President among these voters. Unemployment is high, a little above the national average. There are pockets where layoffs have sent joblessness over 15 percent, like the towns of Decatur and Danville in central Illinois. And coal mine shutdowns have turned the recession into a depression to many in southern Illinois.

Many blue-collar workers in Illinois left their usual home in the Democratic Party in 1980 to vote for Reagan. They were part of a coalition that allowed Reagan to win the state and its 26 electoral votes by more than 375,000 votes.

Now they are moving back in droves, spurred by growing unemployment in their ranks.

It is the rejection of Reagan by these workers that should cause the President the most alarm.

Should this dissatisfaction hold (and it likely will unless the job outlook improves), Reagan could lose an essential element of the coalition that elected him in 1980.

The anti-Reagan sentiment has also been growing of late on the state's college campuses, such as the sprawling University of Illinois at Urbana-Champaign. It is fueled by the feeling among many students and faculty members that the President is following a belligerent policy toward the Soviets that might lead to nuclear confrontation.

In the wake of the Reagan speech at Eureka College, in which he proposed a joint US-Soviet cutback in nuclear missiles while sounding a conciliatory note, there are already reports that some of these academics are taking this as a hopeful sign that the President is moderating his position.

Blacks in Illinois, as elsewhere, remain disenchanted with the President. Despite his visit with a black couple outside Washington, D.C., and his chat with black youngsters in Chicago, the word in the large black community on Chicago's south side, as one black politician says, is ''that Reagan just doesn't care about us.''

Says a black employee at the Conrad Hilton Hotel: ''I'm not even going to look up if Reagan passes down this hall.'' Said another: ''What has he done for us?''

But Mr. Reagan still somehow pulls out ahead among the electorate in this statein polls?, even against the backdrop of what some of his critics call a gathering storm.

In Belleville, in southern Illinois, an observer has just talked to a number of businessmen. They are ''hurting,'' he reports, because of the economic slump. ''But they all still swear by Reagan,'' he adds. ''They still think he will turn things around.''

''If you're working and were for Reagan, you still are for him. If you now are not working, you are against Reagan even if you were once for him,'' says a Chicago barber. A farmer in Eureka said just about the same thing. And a housewife in Springfield said: ''We're not doing as well as we were. But my husband still is working, although not full time. We still haven't given up on Reagan.''

Farmers, particularly those who grow corn and soybeans and raise hogs in central Illinois, are feeling the economic squeeze as they seldom have before. Although many are upset with Reagan, those who till the fertile Illinois soil have generally been loyal Republicans since the party first nominated Abraham Lincoln in 1860. They may complain, but observers feel that the recession would have to deepen to something much worse to cause farmers here to forsake their party and vote for a Democrat -- as they did for Franklin D. Roosevelt in his landside election of 1932.