A funny thing happened on the way to a great economic debate in the 1980 presidential election: The two candidates backed into each other.

After Ronald Reagan's considerable retreat last week from the most radical economic proposals of his primary campaign, he and President Carter are now largely indistinguishable on big economic issues.

Reagan still promises larger individual tax cuts than Carter has, but his position on tax cuts than is less precise and less radical than it was previously, because Reagan has now said spending restraints and tax reductions should proceed simultaneously.

In any event, Reagan has signaled his willingness to accept the essential elements of the broad national economic policy, that has prevailed in this country since the last years of the Eisenhower administration.

To the extent Reagan still proposes new departures, they sound like the campaign-season prognostications typical of presidential candidates who are challenging incumbents.

In 1976, challenger Jimmy Carter adopted a line remarkabily similar to Reagan's this year. Carter then promised tax cuts and new government spending that he said would stimulate the economy to grow again at a rate of 5 to 5 1/2 percent a year, reducing unemployment to 4 percent. That kind of growth, Carter said then, would expand federal revenues so much that by fiscal 1981 (which begins Oct. 1), the government could have a balanced budget and still spend an additional $60 billion on domestic needs.

This year Reagan is making similar promises, though he would spend his growth dividend on defense instead. Cynics may predict that Reagan's promises will produce the same result as Carter's (that is, not much of one) if he is elected president.

Though he has talked for years about reducing the size of the federal government, candidate Reagan is avoiding promises to eliminate or even substantially to cut any existing federal program. The new Reagan position outlined last week is that federal spending should continue to absorb the same percentage of gross national product that it takes today, about 22 percent.

In 1959 government spending represented 19.5 percent of the GNP. In 1976, the last full year of the Ford administration, that figure reached a high of 22.6 percent, inched down in the first years of the Carter administration, and began to creep upward again this year.

As well as any, this statistic demonstrates the long-term continuity of government economy policy. In fact, the principal indexes that reveal how big government spending really is, and how much the federal government takes in taxes from the public, all suggest that years of debates and alarms over economic policy disguise a striking continuity in modern times.

And as the 1980 presidential campaign begins in earnest, the major party candidates are placed within the boundaries of traditional economic policy. Reagan still offers some substantial departures from the Carter approach, but he does so only in areas which, as president, he could not control. Congress would have to approve Reagan's tax cuts or his increases in defense spending, for example.

Reagan continues to talk about wasteful government spending, but his decision not to specify any program that deserves elimination or significant reduction suggests that he has no serious hope of taking such steps. If he could be elected president with a mandate to do away with specific types of federal spending, Congress might feel pressured to go along. But Reagan isn't even asking for such a mandate.

Instead, Reagan is proposing big savings by eliminating "waste" and inefficiency, exactly the line Carter adopted in 1976.

Reagan's latest shifts on economic policy could deprive the Carter campaign of many of the arguments it once thought would be most potent against the Republican. In his acceptance speech at the Democratic National Convention last month, for example, Vice President Mondale charged that Reagan's promised tax cuts would bankrupt the government or force the dismantling of most of ist social programs.

That contention was always debatable, but after Reagan's latest revision of his position, it is indefensible. Reagan now favors smaller tax cuts and promises more savings.

The modern American economy allows plenty of room for politicians to make future promises, because the combination of economic growth and inflation produces steady increases in the size of the pie at stake.

Growth produces real increases in federal revenues. Inflation produces both phony and real increases -- phony ones to the extent that revenues keep up with the basic rate of inflation, but real ones when inflation pushes taxpayers into higher brackets and then takes more of their money.

According to estimates made by the Senate Finance Committee, tax revenues will go from $610 billion in 1981 to $1.1 trillion in 1985 just by letting the economy continue on what amounts to automatic pilot. (Already legislated tax increases in the form of higher Social Security taxes and the "windfall profits" excise tax on domestic oil production contribute substantially to this stunning growth.)

But if recent tradition is followed, the politicians in the White House and Congress will connive to return some of that money to the public in the form of tax cuts, no matter who is elected president on Nov. 4.

If history is any guide, the politicians will keep the federal income tax level at roughly the level it has been for more than a quarter century -- about 12 percent of disposable income.

They will also find ways to spend some of the additional revenue, but future political fashion -- not the rhetoric of this fall's campaign -- is likely to determine who will benefit.

The one economic problem generally regarded as most serious -- infation --is certain to continue, judging by theeconomic forecasts made by the major party candidates.

Indeed, among the increasing number of similarities between their economic proposals might be added one more: Neither Carter nor Reagan is running on a specific set of proposals to reduce inflation. Both proclaim that the problem will diminish as the economy prospers in the future, but neither proposes specific new measures to encourage this to happen.