He was the media darling of the 1976 convention, the telegenic apostle of lowered expectations who stayed in a seedy New York hotel and advanced the radical notion that politicians should also sacrifice.

Coming into the Democratic National Convention four years ago, latestarting Edmund G. (Jerry) Brown Jr. had defeated Jimmy Carter in a string of primaries. Cameras and campaign groupies followed the California governor everywhere. Though Carter had clinched the presidential nomination long before the convention, there were those, even in the Carter camp, who thought of Brown as the wave of the Democratic future.

All that has changed now. Brown is staying in better quarters at a friend's apartment in Manhattan, and his staff is at the Waldorf Astoria. He is all but ignored by the national press, and he is working his way back in the pages of California newspapers as well.

In his home state, he is the only prominent politician with a lower favorability rating than President Carter has. Few politicians seek, or even desire, his endorsement.

And yet Brown, who this year spent $3 million on a presidential campaign that produced one delegate, has not been without influence on his party. His talk of lowered expectations is echoed by many Democrats, sometimes including Carter. Brown's espousal of "reindustrialization" of American cities has become a battle cry of Sen. Edward M. Kennedy. Brown's skepticism of nuclear power and his preference for alternative energy sources is close to being mainstream Democratic dogma.

But Brown has put no political patent on his most challenging ideas, and he no longer seems the wave of anyone's future but his own. His ideas have become important, while he has not. At 42, he seems to many a washed-up prodigy who lacks the means of visible political support.

Even Brown's fiercest critics acknowledge, however, that he is one of the brightest men in American politics. He is bright enough to know what has happened to him. His aides know it, too.

"The missing ingredient has been the lack of political rapport with major political figures in his party," says his chief of staff, Gray Davis. "He'll never be a locker room politician, but he now intends to spend some time with peers within his party."

When Brown was asked by a reporter at a recent National Governors Conference what he had learned from the 1980 campaign, he replied succintly, "I learned that structures were more important than I thought they were."

A friend of Brown said, "Jerry learned you can't be a complete outsider and win a nomination. It's okay to have new ideas and organize new constituencies, but you've got to do it from inside the tent, not outside."

At the convention, Brown is trying to go inside. He attended a breakfast of influential fund-raisers organized by an official in the Democratic National Committee, Charles T. Manatt -- the kind of gathering Brown spurned four years ago. He has given no interviews and issued no news releases.

What Brown is trying to do is accept quietly the lesser role the Carter campaign has in mind for him -- joining his father, Pat, in a sort of family truth squad that will tell Californians how much better life was in the Golden State before and after Ronald Reagan was governor.

In the early primaries, Brown described President Carter as "a failure . . . who was pursuing discredited Democratic policies." He had said that the "Carter-Kennedy wing of the party" was taking the nation "down the road to bankruptcy."

Tonight, Brown's speech to the convention began with a compliment to Kennedy and ended with a commitment to Carter's reelection campaign. He has told Carter aides he will campaign this fall wherever the president wants him to.

Brown sounded a familiar theme tonight when he said that the United States "can learn to place quality above quantity and caring above consumption." But his central message was praise for the president who had defeated him.

"We have a nominee tonight who has broken new ground," Brown said to a convention audience which received him with polite applause."President Carter has taken the first steps toward redirecting American foreign and domestic policy. . . ."

Despite his youth, Brown will not find it easy to come back. He is neither well-liked nor trusted by other politicians. His influence can be measured by the fact that his single delegate here ignored his instructions to vote "present" during the key rules fight, and sided with Carter instead.

Coming to New York in search of what his aides called "a low silhouette," Brown has kept his so low to be almost invisible.

What happened to Jerry Brown, says a Democrat who knows him, is that he reached the top so easily he did not properly appreciate what politicians are supposed to do when they get there.

By toiling in the political vineyards in behalf of the president he scorned, Brown may learn this needed lesson of humility. At 42, it could be a mistake to write him off.