In Des Moines on the Saturday night of Jan. 5, 1980, Walter Mears of the Associated Press asked the assembled Republican presidential candidates how they proposed to balance the budget while reducing taxes and increasing defense spending. Mears' good question drew only one semi-memorable response from the panel: "It's simple; you do it with mirrors," spoke Rep. John B. Anderson (R-Ill.).

Twenty-three months later, John Anderson, having departed the Congress after 20 years and the GOP after 58, finds that earlier harsh judgment of what has come to be called Reaganomics "more confirmed daily" by economic reports. "I did not need the published confessions of Dave Stockman to tell me that the intellectual firmament of Ronald Reagan's program had cracked," said Anderson this week.

Anderson, whose National Unity campaign won 7 percent of the national vote and eligibility for federal matching funds, was back in Washington for lunch at Mel Krupin's restaurant, there to be greeted warmly by a crowd far more conversant with the problems of a three- horse parlay on Saturday than with the prospects for a third party in 1984. The white- haired independent freely and enthusiastically offered his opinions on everything from policy to politics, including along the way his reviews of the 1981 performances of Ronald Reagan, the Democrats, Walter Mondale, and even his ex-House staff assistant and now the nation's budget director, David Stockman.

Anderson remains proud of his unconventional 1980 campaign, to which 230,000 citizens wrote a personal check. (By contrast that year, the Democratic National Committee could count just 75,000 contributors). One of the very few matters about which Anderson appears genuinely undecided is what he and his party should do for and in 1984.

But lest anyone mistakenly think he has retreated into the candidates' sanctuary of "no comments," here are a few samples of updated Anderson differences, delivered in a way that occasionally reminds the listener of a radio commercial for Thirty Days to a More Powerful Vocabulary.

On the administration's economic program: "The Reagan policies run the serious risk of a breakdown in our social consensus. Americans believe in equity and will accept rough times provided there is a perceived sense of equity in the suffering. That is not now the case," according to Anderson.

On the peace or anti-nuclear movement. "President Reagan has done more for the peace movement in just 10 months than anyone else could have." In his frequent appearances before university and professional groups, Anderson sees "an increasing number of people who can be stirred by appeals to peace and against the folly of 'limited nuclear war.'"

On the Democrats and former vice president Mondale. The widespread disenchantment with recent federal programs leaves the Democrats "unable to deny their progeny" and relying almost entirely for their comeback on the failure of the Reagan policies. Mondale is not un-gently described as "shedding publicly the incubus of Jimmy Carter" while seeking the "intellectual purgative which will cleanse him." With lines like that, Anderson can make William F. Buckley Jr. sound like Will Rogers.

He feels a certain bond with the president on the matter of David Stockman and magazine articles. Not fondly does Anderson recall Stockman, who was executive director of the House Republican Conference when Anderson was its chairman. "I was struck with a sense of d,ejMa vu when I first heard about the Atlantic Monthly piece. Dave did something very similar to me."

According to Anderson, Stockman, without informing his employer, wrote a piece for the magazine Public Interest in 1975. That was when Stockman was preparing to run for Congress for the first time. In the article, Stockman "savaged practically every effort government had made to improve the lot of people." Anderson had believed until the magazine came out, that Stockman, who had never confided his doubts in private, had shared his beliefs in those legislative efforts. It is Anderson's judgment that the president believes sincerely in his own economic program and that he must, too, have been similarly surprised and disappointed.

In 1980, John Anderson never won a single primary. But he did win a following. Pollsters insist that there is a growing body of voters who regard themselves as liberal on cultural or life- style issues and conservative on economic issues, a total turnabout in only 12 years. If there is such a constituency in 1984, then John Anderson will have the first crack at it. And he shows no signs of trimming.