SEIZED WITH a passion to pass a tax cut before the election, the Senate would be wise to consider briefly a warning from Congress' own economists. The Congressional Budget Office, in its midyear assessment, points out the basic dilemma. The tax cut that is designed to reduce unemployment in the short run -- the kind of tax cut toward which the Senate is now rushing -- would be inflationary. Conversely, a tax cut designed to restrain inflation by encouraging investment and productivity will take effect only very slowly. Temporarily, it might even make unemployment worse.

Ever since World War II, American economic policy has tried to meet recessions by getting people to spend more money. That's the purpose of the tax cuts and the increased federal budgets -- to put more money into people's pockets, pumping up consumer demand that in turn creates jobs. But, as the recession passes and the recovery gets under way, the high demand generates more inflation.

One necessary remedy for inflation is higher investment in production and productivity. The diagnosis is that Americans are spending too much of their income on current consumption, and too little of it goes into the savings that finance future economic growth.

"The length and depth of the current recession hinges to a large extent on the future course of the saving rate," says the Congressional Budget Office. The saving rate has been extremely low for the past year. If people now begin saving more, they will have to spend less -- prolonging the recession. If they decide to save less, that means an earlier end of the recession -- it also means less investment in future industrial capacity, and higher inflation a couple of years from now.

The real issue lies well beyond this recession. With hundreds of thousands of auto workers out of jobs, the senators want to use the tax bill to increase the customers' spending on cars to get those assembly lines moving again. But ought public policy be aimed at encouraging Americans to buy 10 or 11 million cars every year? A tax policy designed to raise productivity and lower inflation in the mid-1980s would mean less current spending by consumers -- on cars, among other things. It would require a transition to an economy in which fewer people made cars and, for example, more made machine tools.

The question is not whether there is going to be a tax cut. There will be, almost certainly, a cut effective next Jan. 1. The present debate is whether to enact it now, as the Senate wants, or -- better -- next year. Why better to wait? Because it is important to work out legislation that can help get people back to work without running the inflation rate up to 20 percent a year. Any bill Congress tries to write in the few weeks left of this year's session will be another mere speed-up bill, like all those before it. There are more foresighted and intelligent ways to meet the dilemma. They will take more time.