IF PRESIDENT REAGAN and Congress can't cut the budget deficit by more than the $23 billion already assured, that will be a spectacular confession of political incompetence. People in the financial markets will certainly read it that way, and an incompetent government is not a reassuring sight to investors already badly shaken by the drop in stock prices. But the job of managing the budget is not as hard as the negotiators have been making it.

The deficit has to come down, but that's not all. Throughout the economy Americans are consuming more than they can afford and saving too little. It's time to raise consumption taxes, and not just on beer.

The case for a gasoline tax is stronger than ever. It would not only raise the revenues that the government desperately needs. It would also put a useful restraint on the rate at which this country is burning oil. Oil imports have been rising steadily for two years. With the world's chief source of imported oil threatened by an unending war in the Persian Gulf, it's mindless simply to let the country's dependence keep drifting upward.

Each penny per gallon added to the gas tax would raise $1 billion a year. How about a 30-cent tax, phased in over three years? It would do wonders for the deficit. It would also do wonders for the atmosphere in the stock market. People there fear that even if the White House and Congress manage to produce a reduction in this year's deficit, it will be a patched-up list of one-shot gimmicks and nifties like asset sales and accounting changes that promise only the most illusory improvement. A gasoline tax scheduled to rise over time would, in contrast, be a solid promise of real and permanent progress.

As you would expect, the antitax theorists and supply-siders have begun to remind a nervous Congress that attempts to balance the budget in the early 1930s made the Depression much worse. There are a couple of night-and-day differences between circumstances then and now. The American economy, and the world's, was already several months into a recession when the stock market crashed in 1929. Now, in contrast, the economy is expanding rapidly. In the 1930s, the Federal Reserve mistakenly tightened the money supply and raised interest rates. Currently, one crucial benefit of a lower deficit would be to provide the Federal Reserve greater latitude to lower interest rates. That will do more to avoid a recession than trying to persuade Wall Street that big deficits are good for it. The gas tax is too effective a remedy to ignore.