In 1984, Ronald Reagan became the third American president in the last quarter-century to win another White House term in a landslide. But his victory was totally different from the Johnson and Nixon triumphs of 1964 and 1972, when the voters rejected the presidential challengers -- Barry Goldwater and George McGovern -- a lot more than they revered the presidents who won. In 1984, the American electorate harbored no serious doubts about the stability or the intellect of Democrat Walter Mondale, whom they did not vote "against." No, among the president's supporters on Nov. 6, more than six times as many indicated that they voted "for" Reagan as said they voted for the incumbent because they did not like Mondale.

Still, when a Democratic national campaign wins just over two out of five American ballots and loses just under 98 percent of the electoral votes, it's reasonable to conclude that, along the way, the losing Democratic campaign made a couple of mistakes, beyond putting the wrong color on the bumper stickers. The single, biggest mistake made by candidate Mondale and his campaign was the failure to endorse and to embrace the Fair Tax plan of Sen. Bill Bradley (D-N.J.) and Rep. Dick Gephardt (D-Mo.).

There was a strong case to be made, and it was made, for Mondale to support the base-broadening, loophole- closing, rate-lowering Bradley-Gephardt plan, which promised if not to soak, at least to rinse the rich. In October, two-thirds of the voters believed that regardless of who was elected president, taxes would have to be raised in order to reduce the deficit. At that same time, the principal reason given by voters for not voting for Mondale was their expressed concern that he would raise taxes almost as a first resort.

Between one-quarter and one-fifth of the nation's voters -- both distressingly high scores -- then believed that the Democratic nominee was too much influenced by labor unions and that he was the captive of special interests. To complete the indictment, in the primaries Mondale had been tagged as a "candidate of the past." The Democrats urgently needed a "future" issue that would allow Mondale to seize the campaign offensive and to rebut these serious negatives.

Then, as now, no demographic or geographic (or telegraphic, either) group favored raising taxes. But Mondale could have grabbed the offensive by advocating fairness while asking: Why hasn't the president, in four long years, ever done anything to eliminate those flagrant loopholes that were written at the behest of the Very Rich? The position of no cuts in Social Security and Medicare was then only slightly more appealing to voters than was the favoring of a new tax system so that wealthy individuals and corporations would pay a bigger share than they do now. And, of course, by pledging not to cut Social Security benefits, the president could neutralize that sensitive issue. Which, of course, is what Reagan did do.

By publicly emphasizing the Bradley- Gephardt plan, Mondale could have won middle-class support, taken on the special interests, helped his own struggling candidacy and, most important, made tax simplification and tax fairness a Democratic issue. It would have been a valuable legacy and advantage to leave his party. As it is, the ever more urgent issue of tax fairness belongs to neither party right now and can be claimed by Reagan for the Republicans.

Whether it was, as some of his loyalists insist, chronic caution and confirmed habit that prevented Mondale from acting boldly or the Minnesotan's conviction that a $60 billion tax increase (for which Bradley-Gep was imperative, the Democratic nominee did not follow the advice of many of his top political people. It was a large mistake in the campaign year of 1984, which looms as an even larger mistake in the critically important political year of 1985, when Ronald Reagan has the options and the Democrats have only a fragile hold on an issue that might have proved their commitment to the nation's middle class and to economic growth.