IN A REVERSAL of usual presidential campaign practice, challenger Walter Mondale has come forward with a budget plan more specific than that offered by incumbent Ronald Reagan. As public policy, Mr. Mondale's attempt to be specific should be applauded. As a campaign tactic, the wisdom of the declaration will depend on the voters' capacity to tolerate unpleasant truth.

Normally an aspiring candidate has the luxury of promising voters many gratifying but mutually incompatible benefits if he is elected -- and through his primary campaign, Mr. Mondale surely came on as the great promiser. The incumbent, on the other hand -- burdened by analyses, data bases, experienced advisers and the lessons of his four years in office -- usually feels obliged to point out the inconsistencies in his opponent's claims and to spell out his own plans in some detail. Mr. Reagan, however, continues to use the promise-them-anything approach that served him well in the 1980 race, leaving his challenger to deal with harsher realities.

Mr. Mondale's plan, unlike the administration's official forecast, is based on an economic projection that is, at least, internally consistent with what he proposes. Like the administration, Mr. Mondale looks for steady growth and declining interest rates. Unlike the administration, he recommends a fiscal program that makes it somewhat more likely that those things might happen.

The specifics of that plan are strongest on the tax side. Hikes in both individual and corporate income taxes would raise $85 billion in 1989, roughly a third of the projected deficit for that year. A modest amount of the new revenue would come from loophole closings and other reforms, but, like the president, Mr. Mondale leaves basic tax reform for separate consideration. Under the plan, families earning between $25,000 and $60,000 annually might suffer some losses, but the brunt of the new taxes would be felt by families at higher income levels, in other words those earning more than $60,000 a year.

Although people in this bracket were and will remain the big gainers from the 1981 tax cuts, and although, thanks to rising payroll taxes, lower-income people have been shouldering a growing share of the tax burden, it seems doubtful that the affected better-off voters will like what they hear. We expect this will have an adverse effect on such voters, even though Mr. Mondale has explicitly earmarked the proposed new revenue for deficit-reduction rather than new spending.

The spending side of the plan highlights the similarities rather than the differences between the two candidates. Mr. Reagan would surely be glad to underwrite -- and make still larger claims for -- savings proposed by Mr. Mondale in medical cost control, farm subsidies and "management reforms." Mr. Mondale would continue to increase the amount of money the military spends each year, but at a slightly slower rate. Most of the $25 billion in Pentagon savings would be transferred to nondefense programs.

Perhaps, having hazarded so much in outlining his tax-raising plans, Mr. Mondale is to be forgiven for his vagueness about spending cuts. He is not, after all, in office. And the man who is refuses to be specific at all.