RUNNING FOR the Republican presidential nomination early in 1968. Michigan Gov. George Romney did himself in when he admitted he had been "brainwashed" into supporting the Vietnam War. Seeking the Democratic nod for the Oval Office four years later, Maine Sen Edmond S. Muskie permanently crippled his front-running campaign when he publicly wept in response to spurious charges about his wife leveled by the tasteless Manchester Union Leader.

Much of the same sort of self-destruction has happened in Illinois this fall A nip-an-tock gubernatorial campaign has become a runway for Republication incumbent James "Big Jim" Thompson, in the wake of a glaring gaffe by Democratic challenger Adlai E. Stevenson III. Addressing reporters after a debate with Thompson in September. Stevenson complained that the incumbent "is saying me tough guy. as if to simply I'm some kind of wimp." The local press latched on to the remark as indicative of Stevenson's aloofly intellectual bearing, and a series of "wimp jokes" ensued. ("What does a wimp's read on vacation? Adlai Stevenson's economic plan." What's a wimp's favorite drink? Perrier Light.") Thompson--whose populistic slogan is "Tough times demand a tough leader"--capitalized on "the wimp issue" and has coasted from a narrow deficit to a thumping 19-point lead in the most recent poll, released Sunday. When the results roll in tonight from the Land of Lincoln. Thompson, 45, will almost certainly cruise to an unprecedented third term in the governor's mansion that the challenger's father. Adlai E. Stevenson II, occupied from 1949 to 1953.

Thompson's rebound demonstrates how a skillful campaign can gain ground by exploiting an opponent's image problem. The challenger, a former two-term U.S. senator who has never lost an election, seemed early on to have everything going for him. Unemployment statewide had jumped from 6.2 percent five years ago to 12.3 percent today, making Illinois' jobless rate the second highest of any Industrial belt state (Michigan's is highest). Thompson had long embraced Reaganomics, only to see it backfire by sending state debt and jobless totals soaring.

To top it all off, Thompson seemed to be having image problems of his own. Elected first in 1976 to a rare two-year to fill a vacated seat, Thompson focused that campaign on his image as a "Mr.Clean" crime-fighter, and his victory margin of 1.4 million votes helped carry incumbent President Gerald R. Ford to a slim victory over Jimmy Carter in the state. But his image and popularity were tarnished by a series of revelations: he had accepted lavish antiques and potentially compromising campaign gifts in a time of fiscal austerity he had used campaign funds to pay family vacation and baby sitting expenses, he had sought to get his wife a federal judgeship.

YET UNTIL RECENTLY, Stevenson's campaign was so lackluster that Thompson could escape almost all flak for his foibles. Even the traditionally Democratic AFL-CIO only reluctantly bucked his candidacy, and a significant bloc of labor leaders defected to endorse Thompson, saying the pro-Stevenson APL-CIO backing "didn't really reflect the feeling of large numbers of working people." So it wasn't surprising that the restful and resourceful Thompson, who is everything on the stamp that the professional Stevenson is not, stepped in to set the agenda for the contest.

That meant focusing attention on the personality of his eggheadish foe, while putting just a little distance between himself and Reagan. Like his famous father before him, the Illinois Democrat lacks a personal touch. He has compiled his economic program for Illinois in a 300-page redevelopment pamphlet, yet on the stump he has come across as brainy and out of touch. His defensive response to a flurry of wimp jokes--he launched an ad campaign stressing that he had volunteered for the Marines during the Korean War--only accelerated his electoral decline. So did his outlandish last-ditch salvos at Thompson, like equating the incumbent with the Ayatollah Khomeini several weeks ago.

All this mutual mudslinging and image-making is unusual only in degree, and under ordinary circumstances if would be quite funny--if we weren't talking about a race whose winner is certain to be considered at least a dark-house candidate for the White House in 1984 or 1988.

Thompson has publicly let it be known-that he has aspired to the Oval Office since he was a toddler. Should win today, as seems certain, in a year that may well prove resoundingly Democratic while not swerving from right-wing goals, the Tribune predicts he "will automatically become a prospect for the GOP presidential nomination "Stevenson, should he pull off the political miracle of the year, would have nearly as bright a future. With the family name, the Midwest's most populous bully pulpit and a mind suited to concocting a new Democratic platform for 1984, he would be a force to be reckoned with.

All of which should be no reason for national optimism. Both men have savvy--Thompson's is political. Stevenson's cerebral--yet each conspicuously lacks the other's strong suit. Don't be surprised to hear talk of wimps and antique-toting tough guys in Washington in the years to come.