In an era when Gramm and Rudman reign supreme, and Democrats, no less strongly than Republicans, demand a balance in the federal budget, economist Robert Eisner, a maverick from Northwestern University, says most of his professional colleagues have got it wrong.

Contrary to conventional wisdom, the budget deficit is not the nation's No. 1 economic problem, he says. Nor is a red-ink result always bad policy.

Eisner has written a provocative book called "How Real is the Federal Deficit?" (The Free Press, $17.95), in which he contends that the federal deficit has not been measured correctly, inasmuch as the usually cited numbers make no allowance for inflation.

Eisner's main argument revolves around a truism: Inflation creates a bonanza for a borrower. It's fun to borrow money when you can pay back with "cheap" dollars, as homeowners discovered in the post-Vietnam inflation years.

Government debt, Eisner points out, is no different: Inflation reduces the real value of the bonds sold by the Treasury (and those who hold them take a beating). When the Treasury, like the homeowner, pays off its bondholders with cheaper dollars, the real amount of the debt, in effect, has been reduced.

Eisner produces a table showing that the real value of the net federal debt, in constant dollars, actually declined by $327 billion from 1945 to 1980, whereas the traditional calculation in market-value terms, unadjusted for inflation, gives the opposite result -- an increase of $347 billion.

After 1980, under the combined influence of lower inflation and Reagan's huge tax cut (which Eisner labels Keynesian, rather than "supply-side"), the real deficit began to swell instead of decline, doubling in the first four Reagan years.

Therein, perhaps, lies the weakness in Eisner's book: By his own definition, the present deficit is an honest-to-God deficit, growing, in real terms. Therefore, most agree, it presents a threat to the nation's health.

Traditionalists scoff at Eisner's downplaying of the significance of budget deficits on another score. They argue that, because the government must borrow money to finance the deficit year to year, it must deal in "nominal," and not real, terms, scrounging for whatever funds are necessary to cover the difference between annual income and annual outgo.

Although, in the majority view, government borrowing "crowds out" private investment seeking funds, Eisner boldly asserts that deficits "crowd in" investment because the stimulus to demand overcomes the negative effect of higher interest rates.

Eisner contends that economic policy should be based on what is happening in the real economy, not on deficit abstractions. And like some others, he argues that federal expenditures for real assets such as highways and dams should be included in a sensible capital budget account, and not counted like operating expenses.

"After all, the deficit is a means, not an end. The end is the shape of the economy -- whether it is growing adequately, with a good balance of public and private spending," the economist said in an interview.

Eisner speaks of himself as "one of the few unreconstructed Keynesian liberals left," who take seriously the definition of full employment as a jobless level not exceeding 4 percent.

Applying his philosophy to present-day problems, with continued high unemployment, he said this isn't the time to be making sharp cutbacks in spending.

He is critical of former president Jimmy Carter -- and those who advised him -- for not realizing that, on a real, inflation-adjusted basis, the federal debt declined instead of rising during his presidency.

Viewing it around the other -- and incorrect -- way, Carter and his chief economic advisers opted for tight money and other deflationary measures that triggered a recession, Eisner says. And I think he's right about that.

His feeling is that many economists who now have abandoned Keynesian theory do so because they erroneously concluded that the nation suffered high unemployment during the 1970s despite deficit spending. But, by his accounting, "We didn't have the big deficits we thought we had." Instead, there were real budget surpluses that depressed economic activity.

In his conversation with me, Eisner also needled the present-day crop of Democratic, liberal economists, particularly those at the Brookings Institution, who contend that solving the deficit problem is the nation's top economic priority.

"That's almost the party line at Brookings, and they have a tremendous influence on the popular perception of this issue," he says.

Eisner said the Brookings Institution "is ceasing to be an appropriate liberal vanguard. . . . Many [at Brookings] who are identified with the Democratic Party have recognized that this [fight against budget deficits] is a political crusade. The Republicans are beating them over the head, and it becomes difficult to resist that."

When I asked Brookings' director of research, Alice Rivlin, to respond to Eisner's comment, she retorted:

"That's not true, that's garbage!"

She said that the real national debt may have declined during the Carter presidency, "but that doesn't have much to do with the present situation."

Brookings economists who push for a reduction in the deficit do so not for any political reason, but because they believe that the budget deficit is getting to be too big as a percentage of GNP and therefore "is going to be extremely damaging to the U.S. economy," Rivlin said.

Eisner obviously doesn't share Rivlin's concern. If the Gramm-Rudman benchmarks are carried out, he predicts a serious recession. For example, the Gramm-Rudman target of a balanced budget in 1991 would work out, the way he calculates it, to a true surplus of $100 billion, even with a modest inflation rate of 3.5 percent.

Eisner recently debated these issues with Nobelist Franco Modigliani of the Massachusetts Institute of Technology, who feels that deficits have more of a negative effect on investment than Eisner concedes. Modigliani said in a brief telephone interview that he thinks that, in present circumstances, there is less slack in the economy than Eisner believes. Modigliani cautions that unemployment dipping below 6 percent would have inflationary consequences. In any event, Modigliani said, "I can not believe that, all the time, the more debt [we have], the better."

Eisner himself, it turns out, concedes that the present deficit is "extraordinarily large," and he told me he would be willing to see some effort to reduce it if there were a simultaneous easing of monetary policy at the Federal Reserve system. "That's pragmatic," he says.

But he ruefully admits that he can't get influential congressional Democrats to take up his cause: "Some of the stuff I get from Democratic [political organizations in Washington] sounds like the Republicans attacking Roosevelt in the '30s."

Eisner's views shouldn't be ignored: Some Democrats, especially politicians with presidential aspirations, have strayed from liberal principles, on trade as well as budget issues. As he says, Carter deserves no prizes on economic policy grounds.

But we all can accept Eisner's advice not to become hysterical about budget deficits and still believe that it's critically important to reverse the Reagan course of red ink.