Chicago — These are days when Republican incumbents who support President Reagan are supposed to be in big trouble - particularly in the ailing industrial Middle West.
Why, then, is GOP Illinois Gov. James Thompson - shooting for what in this state is an unprecedented third successive term - leading Democratic challenger Adlai Stevenson III by an 8- to 10-point margin in the polls?
Part of the answer lies in ''Big Jim'' Thompson's vigorous insistence that his state's economic woes are part of the national scene and cannot be pinned on him. Although Mr. Stevenson from the start has accused the governor of being a Reagan ''puppet'' and of presiding over the swiftest decline experienced by any state over the last five years, Thompson is emerging relatively unscathed.
''It appears the national economy will be a less significant factor in this election than many had assumed,'' says James Nowlan, professor of political science at the University of Illinois. ''At this stage Thompson seems to have a fairly decent job-approval rating, and voters don't associate him that closely with the economic doldrums in which they find themselves.''
Another factor in the Republican edge is former prosecuting attorney Thompson's successful effort to shift the campaign's focus, at least temporarily , from a defense of his own record to an attack on Stevenson's record as US senator and former state legislator. The governor ended the second of four scheduled debates by insisting that Stevenson voted for a tax increase 33 times as a legislator 17 years ago. Obviously suprised by the charge and at first dismissing it as irrelevant ''ancient history,'' Stevenson later conceded he had voted for the tax hikes, which he said were needed and did pass.
Money and style are also cited as reasons for Thompson's lead. By the end of the campaign he is expected to have spent $4 million to Stevenson's expected $2 million. Most major labor groups support Stevenson. But Thompson is viewed by many as pro-union and has won the support of the Illinois Education Association. A few days ago he even appeared at a fund raiser for a prominent Chicago Democrat.
Many voters would not have given Thompson high marks for leadership in past years. One political analyst says, ''He has seemed to follow the wind, his only fixed star being a yearly balanced budget.''
But Thompson and his supporters are now casting him as not only a good manager but a strong leader. Thompson's campaign takes the troubled economy head-on by insisting: ''Tough times demand a tough leader.'' Many voters now cite his leadership as the prime reason they prefer him.
''Somehow, image seems to make all the difference these days,'' observes Southern Illinois University political scientist John Jackson. ''Usually an incumbent in as long as Thompson has gathered more liabilties than assets. . . . That's why his comeback is so phenomenal. . . . I've never seen a situation where there is such a contrast between the polls and what I see around me - coal mines are closing and factories are laying off workers.''
Indeed, only a few short months ago, Thompson was the widely acknowledged underdog. At the time there was a blitz of newspaper stories questioning the propriety of his acceptance of costly gifts from influential donors including the Teamsters; the spending of campaign contributions on family travel and baby-sitting; and on the his attempt to get a political judgeship for his wife.
''I think it was that rash of unfavorable newspaper stories (that put him behind),'' says Illinois Republican chairman Donald Adams. ''We were actually surprised the governor wasn't stronger against Adlai from the very beginning.''
Neither candidate expected this to be an easy race. Both harbor known presidential ambitions and place heavy stakes on a win. Neither has had to struggle to make his name known to Illinois voters. Stevenson's late father was a popular governor here as well as a two-time presidential aspirant.
But the campaign styles of the current gubernatorial candidates differ sharply: Thompson tends to communicate points clearly and succinctly; Stevenson, often described as very bright and candid but more patrician and professorial in manner, seems to need more time to make a point. Debate moderators have often had to cut him off in midsentence.
Stevenson, who chose a woman as his running mate and has pledged to include Republicans in his cabinet, says he had hoped to focus almost wholly on his plan for Illinois's economic redevelopment. However, he will now try to shift attention back on Thompson's record and argue that the ''imperial'' governor enjoys being governor more than governing.
With less than six weeks and two more debates to go in the campaign, some political analysts say the time for a turnaround is already past. They note the number of ''undecideds'' in recent polls has dropped sharply.
''I think it would take some dramatic event now to change things - like Thompson stumbling or Stevenson debating the daylights out of him,'' says Southern Illinois's Jackson.