The heroic oratory from presidential hopefuls at the recent California Democratic Conference in Sacramento has suddenly given me a clearer grasp of the trouble with the presidency.

Those who seek it, and sometimes win it, are too easily contented with what you might call the politics of gesture--the big talk that arouses great expectations, never mind the probable result.

You expect that of amateurs unblooded and unbowed by the sorrows of Washington. But what about people like Walter Mondale, who spent many years in the U.S. Senate and four years as an unusually busy vice president under Jimmy Carter?

He wasn't the only writer of oratorical rubber checks in Sacramento. But he should know better than most how rubbery they are. Since his speech nonetheless drew rave reviews, maybe it's a good place to start.

You might have thought that Mondale has been watching recent presidential performance from some remote Patagonian outpost, innocent of mail and newspapers. His speech is a poor advertisement for the proposition that a seasoned political professional knows better than to mortgage his future effectiveness to idle hopes.

Consider these random examples: "Right now, if I were president, I would chop these deficits down by scaling the defense budget to reality . . . slam the lid on hospital costs . . . meet with the Federal Reserve Board and insist . . . on an accord that would permit this economy to undertake long- range sustained economic growth . . . pull management and labor together to restructure basic industries. . . ."

And this for foreign policy: "I'd get on that hot line . . . and I'd say this: 'Dear Mr. Andropov, please meet me in Geneva this afternoon, and let's sit down and . . . bring some easing of tensions . . . and negotiate a verifiable mutual freeze on nuclear weapons.' "

Here is the politics of gesture in full spate. Mondale, as is now habitual, makes it sound as if great political goals were achievable by a kind of energetic finger-snapping--single-handedly, too, if the man in the White House doesn't take naps.

But anyone with the sketchiest command of recent history knows that the wish is seldom father to the act.

Has Mondale forgotten, so soon, that the president he served as vice president came to cut, but stayed to raise, the defense budget?

That the Carter administration also sought but failed to get a legislated cap on hospital costs?

That a president who tries to impose a political strategy on the Fed usually gets his ears pinned back?

That labor thinks management is overpaid, and vice versa, and that the harness to hold them both hasn't been designed?

Does Mondale actually believe, moreover, that a quickie summit would yield that elusive agreement on arms control? Or quell the East-West tensions that have plagued the world for a generation? Does he remember what happened when John F. Kennedy bolted off to Vienna for just such an early and inadequately prepared settle- it-all talk with Nikita Khrushchev? Does he remember the great fallout- shelter scare?

I know; it's only a speech. And Fritz Mondale is the fellow who dropped out of an earlier presidential contest saying he didn't want to spend the next umpteen-hundred nights in a motel room. He must establish his zeal; and the sawdust-aisle Democrats who show up for early-season frolics in Sacramento are the grunts, the foot-soldiery, of campaigns and can't be fed on milktoast.

I know. But if his California speech is a portent, Mondale's campaign (and most others too) will soon slip into the familiar and fateful pattern.

The candidates, intoxicated with the politics of gesture, will raise extravagant expectations, certain to be followed after the election by extravagant disillusionment. Two years into the Mondale presidency, if one is achieved on these terms, still another search for still another miracle worker will be on. That has been the story for a long time.

If the presidency is to break out of the cycle of perceived ineffectiveness, those who seek it must grasp the hollowness of idle gesture, however loudly it rings the applause-meter bells. They will begin by asking what the office is worth when it's bought with rubber checks.