Ronald Reagan's decision to keep William Brock as Republican national chairman after all corrects two widespread misconceptions, showing that Reagan is a better politican than supposed and that his political operation may be worse than its severest critics imagine.

Shortcomings of his political high command were revealed in unamimous advice that he needlessly outrage party moderates by dumping Brock. Besides failing to recognize these consequences, Reagan's senior advisers were guilty of naivete. While dumping Brock, they were trying to hire his closest friend and associate, William Timmons.

In overriding this advice, Reagan is not the marionette liberals take him for but a politician who covets the presidency. "I never appreciated before," one moderate Republican deeply involved in the Brock affair told us, "how much Reagan wants to be president." That lesson has been learned during the past two years by longtime associates heaved overboard by Reagan as excess baggage.

These lessons are intrinsically more important than who occupies the office of national chairman, which is mainly intramural politics. Although Brock's ability either to hurt or to help Reagan's campaign is minimal, his fate was closely watched by polticians as a test of Reagan and his campaign staff.

Whatever they may say now, all Reagan's senior staffers urged him to renege on his month-old promise to retain Brock. Apart from failing to anticipate the firestorm within the Republican Party, their ineptitude is reflected in the simultaneous effort to enlist Bill Timmons as Reagan's national politicial director. Timmons was invited to California the same week Brock was to be administered the coup de grace.

That would be asking Butch Cassidy to join the posse before it lynched the Sundance Kid. Timmons, a senior White House aide under former presidents Nixon and Ford and now a Washington lobbyist, was Brock's lieutenant in national Young Republican politics (in the right-wing Syndicate faction) in the early 1960s and later was his congressional aide. The two Tennesseans remain intimate political collaborators.

In Los Angeles last week, two of the Reagan campaign triumvirs -- William Casey and Edwin Meese -- urgently sought Timmons' expertise and prestige (the third, Dr. Richard Wirthlin, was ill.) Regan insiders believe the decisive thrust was delivered by Timmons when he made clear to Casey and Meese that whatever slim prospect they had to getting him to walk away from his business would disappear the moment Brock was axed.

Simultaneously, Gerald Ford, about to undergo knee surgery, told a friend he would consider it "a personal affront" if his endorsement of Reagan were followed by Brock's purge. Party liberals and moderates, never before known as Brockaphiles but looking for an excuse to desert Reagan, declared solemnly that this would alienate them, irrevocably. Early Reagan backers in Congress (Rep. Jack Kemp, Rep. Tom Evans and Sen. Orrin Hatch) pleaded for Brock.

Yet all such advice seemed futile in the face of a contrary position by Sen. Paul Laxalt of Nevada, the Reagan campaign's national chairman in 1980 as in 1976 and a man much beloved by Ron and Nancy Reagan. Backed by such stalwarts as Meese and Lyn Nofziger. Laxalt seemed certain to carry the day. That certainty derived from Reagan's presumed manipulability.

Ideaology was not the issue in the Brock affair. But while ideological strife is declining in the Grand Old Party, factions formerly based on ideology endure. Reagan's choice, in spurning Laxalt's advice, was strickly a political one. Although Brock's foes were interest in avenging old grievances and achieving new party control, Reagan followed a different agenda: to get elected.

This has been the pattern for two years during which Reagan fired trusted aides at the insistence of campaign manage John Sears, who branded them liabilities, and then fired Sears when he himself became a liability. Once Reagan perceived the furor inside the Republican Party that would be set off by Brock's purge, he did not hesitate in keeping him.

This clashes with the myth, held by friend and foe, of a Reagan no better than the men who advise him. If few critics would agree publicly that Reagan is, as one respected reporter recently wrote, "an ignoramus," many would agree in private. The benign side of the indictment is the assumption by many pro-Reagan economists that he would dilute and downgrade his 30 percent, three-year tax cut once they intervened -- and their disappointment that he has not done so.

In sticking to conservative-populist economic views that helped get him nominated and in retaining Brock, Reagan showed that the problem is not Reagan but his advisers. While he staved off disaster in the Brock affair, it enlarged the doubts about the campaign's readiness for Jimmy Carter.