The Illinois gubernatorial campaign is ending, not with a bang but a whimper.

The final debate here Saturday between Gov. James R. Thompson (R) and former senator Adlai E. Stevenson III (D), two of Illinois' brightest lawyers and best vote-getters, was a real downer.

Given the opportunity to question each other directly, Stevenson asked Thompson to justify a $250,000 private elevator in a new state office building and Thompson asked Stevenson why he had attended only a quarter of the meetings of the Senate Banking Committee.

The nit-picking encounter at an O'Hare Airport hotel did not resemble the great debate on the future of the state and national economy that some had expected when the campaign began--or even the spirited personal assaults that marked their exchanges in Peoria and Carbondale earlier in the race.

The fire has gone out largely because most people think the race is over. A 1,500-voter Gallup Poll published last Monday by the Chicago Sun-Times gave Thompson a 54-to-41 percent lead, running even with Stevenson in the city and clobbering him 2 to 1 in the suburbs. A smaller survey, broadcast Friday by the local CBS affiliate, put the state-wide margin at 26 points.

Today, both the Sun-Times and the Chicago Tribune endorsed Thompson's third-term bid, and a veteran Chicago Democratic operative supporting Stevenson said there are widespread reports that some aldermen (City Council members) are making deals to cut Stevenson in hopes of saving others on the Democratic ticket.

Discounting the 26-point margin as "absurd," Joe Novak, Stevenson's embattled campaign manager, said that the challenger's just-starting TV ads and a surge in Democratic registration around the state could signal an upset. But he conceded there are "a lot of mixed signals" being given by the organization of Chicago Mayor Jane Byrne (D), who has donated $300,000 to the Stevenson race but has shared so many platforms with Thompson that she seemed at times to be his running-mate.

The odds are overwhelming that this contest will end the presidential hopes of the 51-year-old Stevenson, who left the Senate in 1980 and sought his father's old job with unconcealed ambitions for the White House. And it will add another notch to the political gun of the 46-year-old Thompson, who began his political climb as the U.S. attorney who sent former governor Otto Kerner Jr. (D) to jail, and who may have presidential dreams of his own.

Unlike his first two elections, in 1976 and 1978, Thompson started behind this time, damaged by the recession, which has given Illinois the second-highest unemployment rate in the country, the scars of six years in office and a spate of unflattering stories about the gifts he received from potential favor-seekers.

In the indelible myth of this election, it will be recorded that Stevenson blew his chances when he complained in a Labor Day interview that Thompson was trying to depict him as a "wimp." The governor, who stands eight inches taller than Stevenson and outweighs him by at least 50 pounds, denied the charge, while happily repeating Stevenson's fatal word. A rash of jokes compounded the problem for Stevenson, who had previously been labeled, with varying degrees of accuracy, aloof, aristocratic, introverted, intellectual and enigmatic.

But according to Phil O'Connor, the Democrat and former insurance commissioner who has run the Thompson campaign, the turnaround came earlier in the summer, and the "wimp" incident was just frosting on the cake.

While Stevenson was vacationing in late July, Thompson reached into his $3 million campaign treasury and began a series of TV ads designed by consultant John Deardourff. They reminded voters that, in a period of recurrent recessions, he had put more money into education, transportation and social services without a general tax increase. They also defended his efforts in economic development, the main focus of Stevenson's attack.

Stevenson attacked on all fronts, accusing Thompson of being a glib salesman who has mismanaged the Medicaid program, shortchanged roads and schools and neglected opportunities to shift the state's economy from declining heavy industry and agriculture to emerging "high-tech" firms. Over and over, he reminded voters that Thompson had been one of the main defenders of Reaganomics and portrayed him as a governor more interested in the perquisites of office than the responsibilities.

It all came into focus in a TV spot by Rothstein-Buckley, Washington consultants for Stevenson, in which the "Illinois employment office" proprietor says to a faceless interviewee, "Sorry, governor, we just can't keep you on. You just haven't done the job . . . . Maybe we can get you something . . . in sales."

Even Republicans concede the humor-tinged message hits Thompson's weaknesses. But it and the other Stevenson spots have been running for only 10 days.

From the beginning, Stevenson has been hampered by his inability to corral the financial and political backing a Democrat normally enjoys in this state. Bothered by his opposition to the Chrysler bail out and the payment of prevailing private wages on state construction jobs, the state AFL-CIO gave him a halfhearted endorsement, but many individual unions continued to help Thompson.

The Jewish community -- normally a major source of funds -- objected to his Senate votes sending sophisticated weapons to Saudi Arabia and gave much of its money to Thompson.

Women's groups failed to deliver on their promise of waves of volunteers and thousands of dollars, even though Stevenson picked a woman running-mate for lieutenant governor against Thompson's choice, the legislator most blamed for blocking ratification of the Equal Rights Amendment.

Byrne and many other organization Democrats, pragmatic as always, continued to play political footsie with the incumbent governor, even though he is of the opposite party.