SUPPOSE THAT a $12 billion jobs program were enacted tomorrow morning, in response to that enthusiastic vote at the Democratic convention. The effect on employment would arrive only slowly, well after the recession had ended. But there would be one immediate effect. Interest would go up, because lenders would assume correctly, that increased spending meant rising inflation. Then a good many Democrats, probably including some of those convention delegates, would accuse the president of imposing high interest deliberately. It's not altogether a hypothetical case. Something very much like it happened last winter.

If the president and the Federal Reserve Board were then to respond to the outcry by pumping up the money supply to get interest rates down the result would probably be a dollar crisis -- a rapid fall in the dollar's value abroad. To rescue the dollar, the president and the Federal Reserve would have no choice but to move rapidly to lift higher than ever, and to cut federal spending. Then the same outraged Democrats would accuse them of breaking the party's pledge on jobs. It is a closed circle of cause and effect, guaranteeing political failure.

The Democrats' platform, as amended, reflects a concept of an economic system that, in reality vanished nearly 10 years ago. Incidentally, the Republican position established last month in Detroit is based on a very similar concept. Ronald Reagan's promised reduction in personal income taxes would have effects on inflation and interest similar to the Democrats' spending on jobs.

What has changed the world? The dollar now floats with no fixed exchange rate. Money moves easily and instantly from one continent to another. Lenders have had 15 years' experience with inflation, and are getting very sophisticated at protecting themselves from it. Under these conditions, the tried-and-true tactics of the 1960s sometimes produce precisely the opposite of the expected results. When larger federal deficits mean higher interest rates, the consequence may well be fewer jobs rather than more.

President Carter's careful statement last night that he supports the "intent" of the platform's jobs language is acknowledgement that he perceives its dangers. There are people in both parties who understand the new logic clearly. But both parties, as they reach for wider audiences, have reverted to the economic formulas that are familiar, accepted -- and obsolete.

Speaking of jobs, it is a politician's job to tell the country where it stands and what it needs to do next. But this summer, both parties have got themselves committed to economic promises designed to bite their own tails.

Perhaps the promises will never be put to the test. The conventions do not speak for Congress. There is absolutely no reason to believe that Congress, under either party's leadership, would pass a foredoomed employment plan at a cost of $12 billion.