Ronald Reagan concluded the first day that the Republican presidential nomination was undeniably his with a singularly uninspired performance pointing to this harsh reality; he is far from ready to take on Jimmy Carter and his resourceful Georgia politicians.

The event here was a $500-a-plate fund-raiser before sleepy-eyed Republicans, including a goodly numer of Sacramento lobbyists, at the palatial Beverly Hills Hotel. Only the day before, George Bush had finally quit. Yet not one word came from Reagan about reaching his once impossible dream of 12 years standing. As he had in two earlier speeches that day, Reagan dispensed the usual homilies; the Beverly Hills faithful stifled yawns.

They would have appreciated hearing the hopes and fears Reagan felt at that moment of triumph (which he confided only to friends at the head table). But no staff aide so advised the candidate; indeed, no such advice was expected. Reagan that night was a lone eagle saying what he felt like saying, just as he had been the last three months. The image of Reagan as the marionette manipulated by clever assistants could not be further from the truth.

From the moment the autocratic John Sears was sacked as campaign manager Feb. 26, Reagan has functioned on his own as major party candidates seldom do. While that did not disrupt his march toward the Republican nomination, it could prove fatal against President Carter.

Important questions have not been answered: Who will tell Reagan not to alienate the blue-collar vote by senseless union-baiting? Who will tell him that proclaiming his unrequited love affair with corporate business, particularly big oil, can only hurt him? Who will convince him to turn the other cheek to Carter campaign attacks on Reagan's record as governor of California, making sure the spotlight stays on Carter's record as president?

Nobody has replaced Sears in fulfillment of these functions. Local politicians who visit the Reagan campaign plane come away worried that his aides seem afraid of him. What frightens them is not their even-tempered chief but the history of bloody staff purges the last two years, unprecedented in a successful presidential campaign.

New York lawyer William Casey, Sears' nominal successor as campaign manager, is far removed from the candidate. At Reagan's side is Edward Meese, a sleek San Diego lawyer who was his chief of staff as governor and whose opinion Reagan values most. When Sears demanded in February that Meese must go, he took on one old surviver too many; Sears was fired instead. But Meese's valued opinions are seldom volunteered. "I would say Ed does not squander his influence," one Reagan insider noted.

When Reagan flirted with disaster by seeming to come out against the minimum wage, Meese did not contradict him; others had to pull him back from the brink. As in Sacramento, Meese on the campaign plane seems more buffer than adviser. His capacious briefcase is becoming famed as the final resting place for myriad memos and position papers sent Reagan.

What Reagan needs for trial-by-combat against Carter is an adviser who is experienced in national politics, has intuitive political skills and is both familiar with and unawed by Reagan. The exact fit for that description is veteran Los Angeles political operative Stuart Spencer, who managed Reagan's first campaign for governor in 1966 but was Gerald R. Ford's chief tactician against Reagan in 1976.

Spencer was more than willing to come aboard the Reagan plane as traveling adviser. But a rancorous debate within Reagan's inner circle included attacks on Spencer that, in the tight little world of politics, inevitably got back to him. By the time word finally was given Spencer that Ron and Nancy would be thrilled to have him back after all these years, it was too late. Spencer had signed on for an independent anti-Carter operation this fall.

Two longtime Reaganites purged last year by Sears are back. Mike Deaver last week began traveling with Reagan for the first time in nearly a year. Lyn Nofziger is signing on as press chief, the job he first held in the 1966 campaign. Both bear the scars of rejection by their chief, but that may reduce reluctance to offer Reagan uncongenial advice.

Reagan often this year has risen above his available advice. Instinctively realizing the political potential of tax reduction, Reagan has rejected pleas by establishment economists that he modify his proposals. He understands that his attack against using unemployment to fight inflation, while abrasive to country-club Republicans, is catnip for Democrats.

But Reagan may be enticed into the political abyss if he lets Carter's experts at negative politics change the campaign's frame of reference to the 1960s in Sacramento. Reagan's automatic response will be to defend his own record, rather than attacking Carter's. Whether a skilled, unafraid adviser is at his side to prevent him from doing so may determine the immediate course of American history.

In a recent column, we inadvertently reported that public employee unions had raised $1 billion -- not $1 million, as was the case -- to fight tax reduction in California.