Only days before the Republican "unity" dinner in Los Angeles June 13, Ronald Reagan faced advice from his oldest, most trusted political associates that he renege on his promise to retain William Brock as Republican national chairman through the 1980 campaign.

The unity dinner was billed as "a laying on of hands" in which Reagan and Brock, at the head table, would confirm each other's legitimacy. But it could result in fists, not hands, considering Republican infighting all too reminiscent of the bad old days of the 1960s.

The advisers who say Brock must go are so influential that it would be a major surprise if Reagan kept him. But Brock's fate is less important than the mess made of handling him. That Reagan as prospective presidential nominee neither firmly retained Brock nor cleanly severed him is evidence that his political operation by no means honed to run against Jimmy Carter.

Simultaneously, Reaganites have been waging a poorly coordinated, consistently unsuccessful campaign to purge anti-Reagan state party leaders. The causes are not policy disagreements but ancient feuds buttressed by cultural differences. Contrary to greatly exaggerated reports of the Grand Old Party united by a passion for victory, these look like the same old Republicans expert at snatching defeat from the jaws of victory. Perhaps fearful of losing to Carter in November, these Reaganites want to consolidate party power now.

When Brock journeyed May 3 to Reagan's California home, such bloodletting seemed likely to yield to good sense in the interest of defeating President Carter. Nobody had ever accused Brock of pro-Reagan passions, but now he conceded Reagan's nomination.

A deal was struck on the Pacific shores. Brock, highly esteemed in the party's non-Reagan establishment, would be retained as chairman through the national election. Reagan would assign his own operatives to Republican national headquarters.

But nobody pinned down the fate of Ben Cotten, the tough Washington lawyer who is Brock's deputy chairman and far less an admirer of Reagan than even his chief. The Reaganites thought Cotten would go; Brock recalled no such commitment. Returning to Washington, Brock began to feel like a figurehead at national headquarters with Reagan's agent, New Hampshire Republican leader Jerry Carmen, in real control.

Oddly, nobody included Sen. Paul Lazalt of Nevada, national chairman of the Reagan campaign, in the deal. Laxalt was outraged that Reagan would not have a national chairman of his own choosing. Carmen last week sent the candidate a secret report urgently recommending that Brock must go in order to remove an anti-Reagan ura at national headquarters. Old Reagan hand Lyn Nofziger, just returned to the inner circle, enthusiastically concurred. So did veteran Reagan adviser Edwin Meese.

Against this array, Reagan is told by the party establishment that the purge of Brock will shatter the gossamer of Republican unity. That view was once supported by Reagan campaign chairman William Casey, but he has quietly bowed to the insistence and persuasion of Laxalt. That is to be made clear to Brock by Casey when they meet in Los Angeles just before the unity dinner.

This is being played out against a comic opera background of victorious Reagan legions reaching for power around the country and falling flat on their faces. The Reagan high command asked that one of Iowa's two national committee members be a Reaganite; that produced a fitful effort to purge former Republican national chairman Mary Louise Smith, which collapsed when Lt. Gov. Terry Branstad and other conservative Iowans refused to cooperate.

A deal in Michigan to elect one Reaganite national committee member ended in double defeat when local Reaganites grasped for two seats. They lost in Colorado and Mississippi, and were too weak to mount a challenge in Connecticut. These effort were neither controlled nor advised from the Reagan high command in Los Angeles.

Thus, on the eve of the "unity" dinner, the Reagan political command showed itself divided, ineffective and irresolute in the Brock affair. Illogically, blundering efforts to purge anti-Reagan members of the national committee left a hard core so furious it might openly oppose the nominee's desire for a new chairman at Detroit. Only Bill Brock's sanity in agreeing to Reagan's wishes as a good party man would prevent such ultimate Republican madness.

That Reagan's political apparatus is not ready to run a national campaign is not surprising. What surprises is the priority on seizing the party machinery, national and state, now in the obvious fear that Reagan may not survive Nov. 4. That Republicans are acting like their old selves is the best political news at the White House in some time.