IT IS ACCEPTED American political practice to applaud the omelet while deploring the broken eggs. But not even a president is entitled to devour the omelet while claiming that it bears no relation whatever to the pile of eggshells in the sink.

Mr. Reagan argues that the decline in inflation over the past 20 months is a triumph of his own economic policy, while the simultaneous rise in unemployment is to be ascribed solely to the unnamed wretches who preceded him. In fact, inescapably, both lower inflation and higher unemployment are the result of the same causes -- high interest and a severe recession. Mr. Reagan would be on firmer ground if he simply pointed out that the improvement in inflation has been a good deal better, at a somewhat lower cost in jobs, than most people thought possible in January 1981.

But the cost in jobs has been substantial, and the trouble with the present position is its instability. Neither Mr. Reagan nor anyone else wants to continue, month after month, with unemployment bumping around 10 percent. Everyone is deeply anxious to see an economic recovery get under way. But the effects of the recession -- on both prices and jobs -- are entirely reversible. Will a recovery bring higher inflation along with lower unemployment?

That's what happened the last time, as the country came out of the 1975 recession. At the beginning of that recovery, inflation was as low as it is today. But the Carter administration underestimated its own program's impact on prices, and within several years they were again trotting upward at dangerous speed.

The trade-off between more jobs and less inflation is as demonstrable now as it was a generation ago. The dilemma of the 1970s was that it took a progressively higher cost in oe to achieve any improvement in the other. The relation between them is not a reliable seesaw, for there have been periods in which this country has inadvertently succeeded in raising both prices and unemployment together. But to make both of them go down together? That's much harder.

The Reagan administration has no brilliant new ideas, but neither has anyone else. The country is now on a track that implies continued unemployment at painfully high levels, as inflation slowly declines. People who press for faster progress on jobs have to acknowledge the inflationary side effects. But similarly, presidents who celebrate lower inflation at their news conferences are not permitted, by the same rules, to disavow the connection with the unemployment rate. That's the nature of the omelet.