-The lesson of the murky 1978 campaign is being flashed repeatedly on television screens across Illinois when Sen. Charles H. Percy appears digging his own grave, paid for courtesy of Democratic challenger Alex Seith.

In a tape from the Percy-Seith debates, the senator declares the American taxpayer is paying a smaller percentage of his income in federal taxes than when percy was first elected to the Senate in 1966. "Isn't that incredible?" asks an offstage voice in the Seith commercial. Not only incredible but, according to Internal Revenue Service figures, wrong.

Just such "negative" advertising - attacking the other guy - Propelled the anonymous Seith into a stunning lead over the supposedly unbeatable two-termer. This is the year when speaking ill of your opponent is smarter than saying well of yourself. Beyond tactics, Chuck Percy's plight shows the immense but largely untapped potential of the tax revolt.

Ironically, obsessive calls for tax-cutting composed the national blueprint for Republican victory, but it was Democrat Seith who followed that blueprint. Not until Percy's frantic managers shifted the spotlight from taxes to Seith's campaign tactics did the tide turn in the polis, pointing toward a cliff-hanger.

Percy, a major figure in his party for two decades, was wedded to the time-tested liberal Republican formula: Vote Liberal enough to attract Democratic support - 13 out of 18 correct votes in 1977 as measured by the liberal Americans for Democratic Action (ADA) - and count on a Democratic foe unacceptable to Republicans. This year, Percy won endorsements from the AFL-CIO, the United Auto Workers and the Illinois branch of the ADA, while retaining big business support.

Percy was warned early by Doug Bailey (Deardourff-Bailey campaign consultants) of tax fever in the air. But Percy is a "problem-solving" Republican not congenial to meatax tax cuts; he delayed publication of a "taxpayer's agenda" brochure, intended to show Percy as a tax-cutter.

Passage of California's Proposition 13 changed that. Percy called campaign aides the next morning, asking: "How's that taxpayer's agenda coming along?" He next endorsed the kemp-Roth tax reduction bill, but when meeting with the superliberal Independent Precinct Organization in August, confided: "There will never be a Kemp-Roth bill, you know that. It will never be enacted into law.

Percy's ambivalence seemingly did not matter. Seith's own Labor Day poll put him 30 percentage points behind. Percy's consultants advised that unless Seith-abandoned by his party and by labor-poured in a half-million dollars, Percy was home free. That sum and more came out of corporation lawyer Seith's and his heiress wife's fortune.

Beginning in mid-September, radio and television spots began assailing Percy's record (including his 1975 vote against Sen. William Roth's first tax reduction.) Seith was so intent on establishing his fiscal conservative credentials that he later wondered whether Sen. Edward M. Kennedy's endorsement of him had spoiled everything.

Seith's anti-Percy barrage was a match to the dry timber of disgruntled Republicans. At a Chicago business establishment breakfast for Percy, one executive greeted us with these words: "Welcome to the wake." Supposedly a Percy booster, he was delighted that "Chuck's finally catching it for playing the liberal game all these years."

When the Chicago Sun-Times straw poll showed Seith ahead in mid-October, the Percy camp moved quickly. Mundane TV spots applauding Percy for opposing Vietnam and Watergate were scrapped in favor of an unprecendented 30-second meaculpa in which Percy admitted, "I'm sure I've made my share of mistakes," but pleaded he had finally gotten the message.

Percy's repentance is skin-deep, as shown in his debate statement suggesting Americans really don't get taxed all that much. In campaigning, he says nothing about Kemp-Roth. What enthuses him is not revenue reduction but revenue sharing, a "problem-solving" Republican initiative.

It was much too late to transform Percy's image. "What we had to do was to change this from a referendum on Percy into a look at his opponent," a Percy manager told us. That became easier after a Seith radio spot on black radio stations outrageously linked Percy's praise of former Agriculture Secretary Earl Butz to Butz' infamous racist joke. The Percy campaign next bought fullpage newspaper advertisements reprinting anti-Seith columns in the Sun-Times by the celebrated Mike Royko.

Percy's late rise in the Sun-Times straw poll indicated the counterattack was working. Yet the need for such expedients is remarkable. The entire establishment - labor, business, newspapers - has backed Percy, whose endorsements run from Ronald Reagan to the Rev. Jesse Jackson. While carloads of senators poured in for Percy, only Lillian Carter came here specifically for Seith.

That Seith could even threaten Percy is testimony to the firepower of both negative advertising and the tax revolt. It suggests lost opportunities for 1978, but points to reach 1980 gains for those who perceive the public's mood.