The low point of Pete Wilson's campaign for the Senate came on a warm summer Saturday when he learned for the first time that he had paid no federal income taxes in 1980.

The date was Sept. 18, 1982, more than a year and a half after the tax return was filed. Wilson already was well on his way to blowing a 22-point lead he had once enjoyed over Gov. Edmund G. (Jerry) Brown Jr., the Democratic nominee, and the disclosure hit him hard.

"It was pretty damn depressing," Wilson said in recalling his discomfiture. "Talk about the worst of all possible circumstances. We had scheduled a news conference based on a law enforcement endorsement. The guy who was supposed to do it had a minor crisis in his department so he couldn't come. We decided to cancel the news conference, which in retrospect was very unfortunate timing. It looked like we were ducking the thing."

The reporters "asked me about it on the steps of the L.A. Chamber of Commerce, where I was going to make a speech. 'What did you know about it?' they wanted to know. I didn't know anything."

The tax story symbolized a summer in which Wilson, the 49-year-old mayor of San Diego, had regressed from the Cinderella winner of a Republican primary to a seeming political stumblebum. Much of the time Wilson seemed to be collaborating with Brown's gut-level effort to substitute his verbal stumbles for Brown's own unpopular record as the chief issue of the campaign.

Now, with little more than two weeks before the Nov. 2 election, Peter Barton Wilson's unhappy ordeal is nearly over. And like many other chapters in an upward striving life in which he has been widely recognized as a competent legislator and effective mayor, this one also may have a happy ending.

After a long summer of discontent and a campaign more flawed than a potholed road, Wilson has pulled even and taken the lead while Brown's heavy negatives have reasserted themselves.

A Teichner poll commissioned by two television stations, the first survey to show Wilson's slippage last summer, last week gave Wilson a 6-point lead. A later tracking by the GOP firm, Decision Making Information, put Wilson 10 points ahead. A Los Angeles Times poll released yesterday gave Wilson a 46-to-41 percent lead.

"We'll stagger in at the wire," said one veteran GOP political operative, who believes that unemployment and a big Democratic vote will make the race close. Brown hopes that economic discontent will give him the narrow victory.

Winning by any margin would be a tremendous achievement for the hard-working Wilson, who in two years has overcome personal and financial problems, the suspicion of Reagan loyalists engendered by his support of President Ford in the 1976 New Hampshire primary, various political blunders and some of the harshest political commercials ever filmed.

The most controversial of these ads was a 30-second spot that began with a film of Leonard Bernstein, Candice Bergen, Tony Randall and other celebrities proclaiming how they want to "go on making music" or "go on acting." They are followed by a nuclear explosion and a child saying, "I want to go on living."

Brown then appears with a U.S. flag, saying: "Pete Wilson opposes the nuclear arms freeze. Jerry Brown supports it. Vote for your life. Elect Jerry Brown for the U.S. Senate."

The ad drew widespread editorial protests even from newspapers favorable to Brown and was quickly withdrawn.

Almost equally controversial, although less publicized outside the state, was an ad that capitalized on what many consider Wilson's most egregious political goof: his statement earlier this year that the Social Security system was in such serious trouble that a variety of remedies, including voluntary participation, should be considered to solve its financial ills. Brown claimed, without further evidence, that this meant that Wilson favored reduction of Social Security benefits.

One widely shown Brown commercial features a woman who proclaims herself as a Republican from San Diego, opposes Wilson for supposedly favoring Social Security cutbacks and says he deserted "our president" on the income tax bill passed this year by a coalition of presidential and congressional leadership. As it turns out, the woman is from Coronado, is registered "declined to state" rather than Republican and has held fund-raisers in her home for Brown.

This sort of advertising, compounded by Wilson's fumbles, steadily eroded Wilson's lead. The California Poll taken by Mervyn Field showed Wilson leading Brown by 22 points in early June and a percentage point behind in a survey released Oct. 8. The Teichner survey showing Wilson 6 points up was taken Oct. 9-10 after a foreign policy debate in which Wilson more than held his own. Private polls indicate a slow but steady Wilson trend ever since.

What is uncertain, however, is the impact of a new attack by Brown, this one based on a $10,000 investment made by Wilson in a tax-sheltered energy conversion firm.

The investment, which Wilson said was intended to shelter real estate income of his wife, was for buying equipment designed to convert cow dung into methane gas.

Brown, whose operatives have been investigating this investment for several weeks, misstated the investment as a "loan" when the two candidates debated on domestic issues last Monday night. Wilson, unprepared for the attack, scored points by telling Brown, with some heat, that if he had any evidence of wrongdoing he should "take it to the district attorney."

But Wilson typically threw away his advantage the following day at a news conference, to which he came unprepared to discuss the investment. It took Wilson another day and another round of stories before he laid out his version of events.

This inexpertise in dealing with Brown's attacks has been typical of the campaign. Some blame this on Wilson, saying he suffers from a tendency to function as campaign manager rather than candidate. Others focus on the inexperience of Wilson's campaign management.

But the whole story of Wilson's summer ordeal is more complicated and personal. Wilson was separated from his wife of 13 years in 1981, and most of the financial decisions concerning his taxes appear to have reflected this separation and to have been made by Wilson's accountant and lawyer without his involvement. He wound up without money after the settlement, living on his annual mayoral salary of $36,000 and accepting free rent from a friend.

In the GOP primary, Wilson started out a distant third to Reps. Barry Goldwater Jr. and Paul N. McCloskey Jr. and overtook them both after a costly campaign. When the primary ended in June, Wilson was an exhausted victor with $10,000 in his campaign treasury, compared to $2 million for Brown.

Despite his victory, Wilson was relatively unknown statewide and Brown proceeded to "define" him for the voters.

"He had to get them to focus on the negatives of 'Mayor Wimpy' rather than 'Gov. Moonbeam,' " said one Democratic operative.

Brown succeeded in the short run, but his negative campaigning may have produced a backlash of sympathy for Wilson.

Though Wilson is noticeably more shy and less aggressive than Brown, he also has waged a calculatedly negative campaign. On the same day that Wilson's nonpayment of taxes was disclosed, the mayor remembers veteran political consultant Stuart K. Spencer, who stabilized President Reagan's campaign when it was faltering in 1980, telling him: "Something strange is happening all across the country. People are believing negatives."

The Wilson campaign has responded to the cue. Wilson calls Brown "Gov. Flip-Flop" and his commercials stress Brown's connection with Jane Fonda and Tom Hayden and his appointment of a U.S. prisoner of war suspected of collaboration with the Vietnamese to the Orange County Board of Supervisors.

As Wilson enters the final two weeks of the campaign, hoping that Brown's 48 percent negative rating will count for more than his own mistakes, his prospects were defined in different ways by two close supporters.

Reviewing Wilson's prospects last week, one longtime friend and strategist said: "I don't think Jerry can beat Pete. But I'm not sure Pete can't still beat Pete."

Bob Goodman, the veteran political adman and jingle-writer who has helped shaped Wilson's campaign, was more bullish on the prospects of his candidate.

"He doesn't have to be Ben-Hur," said Goodman with a shrug. "All he has to do is be better than Jerry Brown."