Maybe it's impossible to talk sensibly about the U.S. economy during an election campaign. Maybe the process of economic change is too obscure and complicated to capture the popular imagination. Or maybe the things that need to be said are too unpleasant for anyone to say who wants to be elected to office. Whatever, the enthusiasm for cliche is almost universal. Republicans and Democrats share equally in the passion.

Those glum thoughts owe their inspiration to Ronald Reagan's acceptance speech as the Republican president nominee. It was grand oratory. It made you want to believe, which is what good speeches ought to do. But Reagan's economics wander across the narrow line that divides rhetorical license from sentimentality. In his view, Republican tax proposals wil be a marvel; they will spur the renaissance of private enterprise, squash inflation, expand employment and revive productivity. Meanwhile, despite lower taxes, defense spending will be higher.

Comparisons are odious, but a comparison with Jimmy Carter in 1976 is almost unavoidable. The constant frustration of economic policy over the past two decades has been the apparent inability to reconcile low unemployment and low inflation. Carter paid little attention to the apparent conflict in his campaign and even less in his first year in office. Now, Reagan seems scarcely aware of its existence. This is an invitation to another cycle of overpromise and disillusion.

Reagan's happy message originates in supply-side economics, a new conventional wisdom embraced by many Democrats as well as Republicans. The basic idea is simple: Effective tax rates have risen too high. They discourage work and investment. Lowering tax rates will spur investment and create jobs. But lower tax rates also make it more attractive for people to work, and the higher supply of both investment and labor will neutralize inflationary pressures.

This is a sensible argument, but no miracle cure. There may be a shortage of some types of investment, but there's no overall labor shortage. Moreover, cutting taxes will be extraordiarily difficult, and the effects of lower tax rates on investment and work habits aren't clear.

Start with the prospects for a real tax reduction, as opposed to a cut that simply offsets inflation's effect of kicking people into higher tax brackets. What's not generally reaized is that federal spending moved to a higher plateau in the 1970s than in the 1960s. In the 1960s, federal spending averaged 19.5 percent of gross national product, the economy's total output; this year, it's projected at about 22.5 percent. With GNP at $2.5 trillion, the difference is worth roughly $75 billion.

And a look at the 1980 budget shows why it will be difficult to change things dramatically. Of $579 billion in estimated spending, nearly two-thirds represents categories that probably will increase or are politically immune to large cuts.

Defense spending accounts for $136 billion of the total; Reagan wants to increase that sharply (after allowing for inflation), Carter a little less so. Another $117 billion goes for Social Security, including disability; these payments are linked automatically to inflation, and the number of beneficiaries will grow. Medicare accounts for another $33 billion -- same basic story. Civil service pensions total $14 billion. Interest on the federal debt now totals $64 billion.

Even a ruthless Reagan with a pliant Congress couldn't reduce spending easily to levels prevailing in the 1960s. That means tax rates will be higher unless the government raises its deficit. Whether that's a good approach depends on how you answer a key question: Will the incentives of lower tax rates outweigh the adverse effects of larger borrowing?

The answer isn't clear. People may work longer or harder if they can keep more of their earnings, but higher after-tax incomes may lead some people to take more time off and work less. Other factors -- population increases, cultural changes -- may swamp the effects of lower tax rates. Between 1975 and 1979, when effective tax rates increased sharply, 10.3 million people flooded the labor market.

The same uncertainties apply to investment. Inflation increases the effective tax rate on business because depreciation allowances for wear and tear on machinery aren't adequate. (Depreciation allowances are based on the original purchase price of equipment, which is far less than the actual replacement cost.) Both Carter and Reagan favor more generous depreciation allowances, but the precise spur to more investment and higher productivity remains speculative.

None of this is an argument against such measures, but simply a caution. Modest benefits of lower tax rates may be offset by adverse effects of larger government borrowing: higher interest rates or an inflationary stream of money from the Federal Reserve.

But caution gets thrown to the wind in political campaigns. The Republicans now seem to have adopted the Democratic belief that the government, through one lever or another, can accurately manipulate the economy's performance.

Reagan as yet hasn't equaled Carter's 1976 extravagances, but that would take some doing. Remember Carter's pledges? He would lower unemployment to 4 percent by 1981 (it was 7.7 percent last month), reduce inflation to about 4 percent (the consumer price index has risen 14.4 percent in the past year) and balance the 1981 budget (the deficit is estimated at $30 billion). At the same time, he was going to cut taxes and adopt expensive new programs such as national health insurance.

Bemoaning a lack of election candor is an innocent, almost naive complaint. If you regard elections simply as a matter of victory or defeat, then the half-truths, exaggerations and simplifications that get peddled are of little consequence. But if you believe that campaigns also serve as the vehicles by which the public is "educated" and forms expectations of government performance, then we are paying a high price in pubic cynicism for election excesses. In a democracy, the ultimate responsibility of pliticians is to maintain confidence in the political system. Campaigns seem to make the job tougher, not easier.