One of Walter Mondale's best issues in his fight against Ronald Reagan is the huge federal deficit. The deficit, most experts agree, results in high real interest rates that threaten the long-term growth of the economy.

But it's an exceedingly hard issue to define for the citizen-voters: the deficit is an abstract concept, especially when the high interest rates people must pay now to buy a car or a home have not thrown the economy into reverse gear. The deficit, in other words, is a danger -- and a real one -- somewhere down the road. But it's not a palpable danger now.

Nonetheless, Mondale followed a pledge he made at the Democratic convention by detailing his plans to cut Reagan's deficit from an estimated $263 billion in 1989 to $86 billion. The major elements include $85 billion in new t revenue (to be placed in a trust fund and used only to cut the deficit, not for new spending programs); and a net reduction of $75 billion in government spending.

Mondale's program is economically sound and responsible. But the political dangers inherent in a gutsy decision to confront the deficit issue are even more apparent now that the details are public. The immediate response from the Reagan administration was that the spoilsport Democratic candidate would mess up the nice recovery that the president had arranged by increasing taxes.

"It looks as if Reagan gives (tax) money away, and Fritz takes it back," moaned a tax expert who is solidly anti-Reagan.

A sample of the predictable administration rebuttal from longtime Reagan political insider Lyn Nofziger: "(Mondale's plan) is nothing more than a recovery-killing concoction which is ready- made to produce a recession that will throw millions of Americans out of work again, pile new and higher taxes on the backs of the average citizen, and make deficits higher, not lower."

Of course, Mondale did not propose raising the taxes of the "average" citizen. Those in lower-income brackets -- well over half of all taxpayers -- would not be affected. The message that Mondale wanted to get across -- but didn't quite put over -- was in two parts.

First, upper-income taxpayers today are not paying a fair share of taxes under Reagan, who (with a willing Congress) provided them all sorts of concessions and special tax favors. So, Mondale's tax-increase program is not across the board, but hits hard at the wealthy and at corporate loopholes and shelters.

Second, the real threat to long-term economic growth -- and a possible new recession -- comes not from raising taxes so as to cancel some of Reagan's giveaways but from the high interest rates triggered by the budget deficits. A budget deficit reduction program as blueprinted by Mondale would allow the Federal Reserve Board to ease up its tight monetary stance.

"The Democrats have to recapture the economic growth issue," says a friendly critic. "Mondale needs a theme. He has to assure people that higher tax revenue will help, not hurt, the economy. The only way to get a sustained recovery is through lower interest rates."

Mondale also boldly attacked Reagan on the spending side of the federal budget, charging accurately that the "government hasn't shrunk under Mr. Reagan, it has expanded" except in programs for the poor. Mondale would restore some of those funds, part of $30 billion he would add to the budget by fiscal 1989.

But significantly (despite whatever promises may have been made to Jesse Jackson) there is no big "jobs" or antipoverty program to deal with unemployment: Mondale is making the more conservative assumption -- a departure from the Great Society approach -- that private-sector economic growth, rather than big spending, will create jobs.

Against that $30 billion in added spending, there would be $105 billion in reductions -- $51 billion in lower Treasury interest payments as the deficit is reduced. The rest of Mondale's spending cuts would come mainly from defense, health, agriculture and other domestic programs.

How good is this program? One weakness is that it sidestepped the need for longer-range tax reform. But it seems an honest attempt to get the economy in a position where the Fed has an incentive to lower interest rates. It would cut some fat out of defense, and put money back into programs for the less advantaged. And it would help complete the process of voiding some of the tax favors Reagan distributed to his friends.

Fritz Mondale's problem is that he hasn't been very good at articulating the philosophy of his own program, and that makes it harder to convince people that it will work. There's time to do that, but it's running out fast.