Against a backdrop of sun-dappled Lake Michigan, Illinois Gov. James R. Thompson slouched on a sofa 18 stories up in the Conrad Hilton Hotel and shook his head about what had just happened to him.

An hour earlier, he had shared the head table with President Reagan at a YMCA luncheon, but instead of basking in the presidential glow, Thompson was embarrassed by the fact that his political misdeeds had become the subject of the president's news conference.

Thompson is reeling from articles in Illinois papers describing how he accepted antiques and expensive paintings from friends and contributors, used campaign funds to pay for babysitting expenses, vacations and clothing, and tried to help his wife gain a federal judgeship.

Since these expenditures were reported, Thompson has repaid his campaign $3,316, publicly admitting his mistake in billing the campaign for certain of the expenses. He also instituted a new policy of accepting no gift worth more than $100.

The revelations have hurt Thompson's bid for a third term. His Democratic opponent is former senator Adlai Stevenson, whose name alone makes him a formidable opponent in Illinois. A Chicago Tribune poll about three weeks ago showed Thompson falling narrowly behind for the first time in the campaign.

Thompson's campaign is being watched with more interest than normal by other Republicans because none of the other midwestern Republican governors whose terms expire this year is running for reelection. Govs. Albert Quie of Minnesota, Robert Ray of Iowa, Lee Dreyfus of Wisconsin, William Milliken of Michigan and James Rhodes of Ohio all have bowed out this year.

After landslide victories in 1976 and 1978, Thompson now clearly faces his most difficult campaign, one in which both the economy--Illinois' unemployment rate is now 10.4 percent, 1 percentage point above the national rate--and Thompson's penchant for gift-taking will be central issues.

"Right now, Thompson's got two opponents--himself and Ronald Reagan," said Don Rose, a Chicago political strategist who helped elect Democratic Mayor Jane Byrne.

When Reagan opened his regional news conference in Chicago on Monday, the first question he got was about the propriety of Thompson taking $500 in cash from a Teamsters official, and he was forced to defend the governor without knowing many of the details of what had happened.

As Thompson later replayed the episode, it was clear he was beginning to understand how costly his mistakes have been.

"It hurts," he said. "It's painful, it's embarrassing. How do you think I felt sitting up there on the dais after I heard about that? Not very good."

That may be an understatement, coming from a man who pledged in his first inaugural: "Impropriety will be treated firmly by the law. The appearance of impropriety will be treated firmly by me."

Then Thompson was still the aggressive ex-prosecutor who had launched his political career as a U.S. attorney who won a string of convictions against public officials, including former Illinois governor Otto Kerner, a Democrat.

But today, although there are no charges of illegality stemming from the disclosures, Thompson's halo is gone.

"What he's done is not offensive to the prevailing standards of Illinois--unfortunately," Stevenson said this week. "But he was holier than everyone else. Now he's just a little seamier than most."

Ironically, Stevenson acts as if Thompson's troubles are an intrusion upon his own campaign.

"I hate to see attention diverted from things I really want to talk about," he said, preferring to concentrate on a blueprint for developing "a post-industrial economy" for Illinois, ideas that, while farsighted, have attracted little attention.

Thompson's campaign advisers say the stories about the gifts and the clumsy effort to obtain a judgeship for Jayne Thompson, which smacked of arrogance to many Illinoisans and may have the most lasting impact, have done less damage to the governor than they expected. They are still nervous about his reelection chances, however.

"The race is essentially a tossup, but that's in the face of as much bad news as you can get in a month or six weeks," said one adviser.

The revelations about Thompson have been coming since last fall, when it was disclosed that his former Chicago law firm had received about $700,000 in state business, which Thompson defended as simple patronage.

But the most damaging involved his acceptance of antiques and six paintings worth thousands of dollars, and his use of campaign contributions for babysitting and other expenses. The Internal Revenue Service is looking at Thompson's records.

Thompson, an antique collector, often set aside articles in one Chicago gallery, which friends later came in and bought for him.

"I know almost every antique dealer in the state of Illinois," Thompson said. "They let you put things on layaway that you can't afford now, that you'd like to have some day. My friends know where I shop. They started coming in and occasionally buying some of those things.

"They were not directed there," he said, adding, "It wasn't a bridal registry," as one magazine called it.

Illinois papers also revealed, based on records kept and freely made available by Thompson, that the governor had used nearly $4,000 in campaign funds in 1980 and 1981 to pay for babysitting expenses and another $11,500 to provide gifts to staff members, campaign aides and contributors.

He also used campaign funds to buy himself and two aides stadium jackets, to replace a pair of shoes, to fly himself and his wife to Florida for two vacations, and to fly relatives to Springfield for Thanksgiving dinners.

The governor said his campaign records have been examined by lawyers and auditors who applied "a very conservative standard" and recommended that he pay for expenses that were "arguably political," but still questionable. He notes that the $3,316 he repaid is only a fraction of the more than $6 million in funds his campaign has raised and spent over the last six years.

He defended much of the money spent for babysitting. "I'm one of the new generation of officeholders with small children," he said. "There's no other way to do it, and there's nothing improper about it." Other Illinois politicians have acknowledged doing the same thing.

Thompson also defended the acceptance of large gifts, including the $500 in cash, arguing that none came in return for or with the expectation of a favor from the governor.

"I think sometimes in these affairs we overlook basic human motivations," he said. "People find out the governor of the state is a human being that you can talk to and laugh with and learn things with and enjoy. It operates on all kinds of different levels.

"You know, if you look at the gift book in which he records all gifts , you'll see the range of human emotions in there. Grandmothers who knitted things when my daughter was born because a child hadn't been born to a sitting governor in 72 years. They were acting out of their emotional feelings. They just did it on a different scale from a guy who was selling paintings."

Did he equate knitted gifts for his baby daughter with expensive paintings from gallery owners or contributors?

"I'm not saying that the two are the same, and obviously I'm paying for the difference," he said. "Before I'm through, I will have paid many times over."