More than 200 contributors to independent presidential candidate John B. Anderson sipped cocktails patiently for two hours at the University Club here Tuesday night waiting for the congressman, but their patience exceeded their commitment to Anderson.

Delayed by a tornado that closed the airport Anderson never did make it to the $100-a-head fund-raising event. In his absence, the basic weakness of Anderson's National Unity Campaign" was ruthlessly exposed in conversations over cocktails and brownies.

"I love John Anderson," one key supporter who helped plan the event told us, "and I'll stick with him another two or three weeks, but then . . ." Then what? "Then, unless it looks as though he might win, I'll go to Carter to make sure Ronald Reagan doesn't become president."

That theme was not universal in the crowed University Club living room, but it was repeated often enough to flash a warning to the 58-year-old maverick Republican. Unless he makes a dramatic breakthrough in his Sunday night debate with Reagan, his campaign may be headed for obsolescence its impact limited to a very few states none of which he will actually carry.

When we asked Anderson about the political syndrome of voters who want Anderson but will accept Carter to stop Reagan, he was philosophical. He conceded the problem but did not advance a solution. What makes it such a problem for Anderson is that he is not an ideological magnet who transfixes his supporters as George Wallace did in 1968. Wallaceites had an allegiance to Wallace that nothing could break, but the appeal of Anderson is based on the unpopularity of Carter and Reagan and Anderson's promise that he can do better.

Morover, there was no sign here that the Anderson camp views his debate with Reagan as a campaign watershed. He told us he will use the hour of nationally televised prime time, a precious asset to a campaign hard pressed for money, to "distinguish" his policies from Reagan's, not to attack President Carter. "If I attack Carter, I begin to look like a pale image of Ronald Reagan and I'm not that, I'm not that," he told us in his car riding across Illinois Madison County to a high school rally at Edwardsville on Wednesday.

"Distinguishing" his position on issues from Reagan may not be just the ticket for a dramatic liftoff from what one aide calls his present "plateau" of around 15 percent in the polls. In fact, on two of the major differences Anderson has with Reagan -- tax cuts and defense -- Reagan has by far the better political position according to public opinion polls and our own interviews with voters. Reagan favors deep tax cuts and higher defense spending. Anderson opposes tax cuts without a balanced budget. On defense, his 39-page Agenda for America" relegates military issues to page 35. The platform pledges only to "spend what we need" for defense, mentioning no figures.

Anderson's antipathy for the military made a bizarre appearance during a press conference at Edwardsville Senior High. Challenged to produce evidence in support of his charge that Carter has turned foreign policy over to the Pentagon generals, he suggested that the generals were to blame for leaking secret information on the Stealth aircraft. In fact the generals were furious at the leak, which seemed to have been plotted by Carter aides seeking political capital from the Stealth breakthrough.

There were other signs of either fatigue or confusion in Anderson's last day of campaigning before Sunday's debate. He told the students at Edwardsville that Israel had "recovered" East Jerusalem from Jordan during the 1967 war, when in fact East Jerusalem never had been part of Israel. He misplaced Afghanistan in Southeast Asia.

Such minor slips of the tongue are excusable. Anderson's problem is not fatigue, monmentary confusion or even lack of money, serious as that is. It is the threat of flight between now and Nov. 4 by his own backers growing more and more worried that their votes may be wasted.