Barry Goldwater, the last conservative to become the Republican presidential candidate, won the nomination after a bitter, personal fight that carried all the way to the convention floor. In the partisan frenzy of the moment, he picked, in William Miller, a running mate who offended the Republican moderates. The result was a historic disaster -- for the country as well as the party.

Ronald Reagan tied up the nomination early this year in a benign campaign that left few hurt feelings. So he has had the chance to pick at leisure a running mate who broadens his appeal. But if he misses the chance, if he does not name one of the three prominent moderates, he reinforces the damaging stereotypes of the right-wing ideologue.

In all fairness, the sterotype may be a grotesque caricature. Reagan, seen on his home field here in Los Angeles, does not come on as a threatening per person. Witness his love of making people laugh with one-liners. Several of his stands -- for example, being against draft registration -- clash with the ahrdline position.

But the moderate Republican vote takes on very special importance this year. For one thing, John Anderson will be in the race bidding for Republican moderates. For another, the crucial battleground lies in states -- Illinois, Ohio, Michigan, Pennsylvania, New Jersey and maybe even New York -- where moderates plainly outweigh conservatives.

Moreover, if the ideological shoe doesnht exactly fit Reagan, there is a certain correspondence on major matters as well as such emotional issues as women's rights. In economic policy, for example, the Reagan stress is on an across-the-board tax cut. That would promote the interest of the haves. Add to that his previous refusal to divulge tax returns, and you have the makings of an economic royalist, soaking the poor.

In the energy field, the Reagan position emphasizes inducing a new burst of oil supply by decontrol of prices. Unfortunately, almost all forecasts -- both private and government -- suggest that reserves in this country have been depleted to the point where even the price incentive cannot stimulate higher output. So the Reagan position lends itself to the critique that without solving the problem, it offers giveaways to the big companies.

In the cloudy world of foreign policy, the telltale issue is China. That country ties up a significant portion of Russia's armed might. It challenges the Soviet claim to supremacy in the communist world. It engages Japan in a tilt against Russia. It is, accordingly, crucial to any serious American hope of checking Soviet expansion.

Still Reagan, while coming off his love affair with Taiwan, shows no real affinity for Peking. Why? Because, the argument can be made to go, he is such an ideologue on the abstract issue of communism.

Join to those issues inexperiece in national and international affairs and age, and there emerges a candidate extremely vulnerable to the kind of piranha politics Jimmy Carter plays with a vengeance. Except that by a rare stroke of good fortune, Reagan can erase the stereotype by picking one of three possible running mates.

George Bush is the obvious choice. As the former director of central intelligence, an envoy to China and the United Nations, he covers Reagan on the questions of experience and age. He has an appeal on the East Coast, whence he hails, and in the industrial states of the Midwest where he showed strong in the primaries.

Howard Baker, although he did poorly in the primaries because he ran from the position of Senate minority leader, is a weightier figure than Bush. fLike Bush, he saves Reagan in the matters of age and experience. He has standing in border states that could be important, and he is a poised campaigner, particularly on television.

Gerald Ford is the dream candidate. He has the quality Reagan most needs in a running mate, the quality of being presidential. Alone among American political leaders, he excites no boos. To be sure, Ford has said no, and means it. But if Reagan made the right kind of acceptance speech -- "The country needs Jerry Ford, the party needs Jerry Ford, and I need Jerry Ford" -- the convention would sweep Ford into the post by acclamation.

No doubt there are other good possibilities. Gov. Albert Quie of Minnesota is an attractive figure, as are Sens. Richard Lugar of Indiana, Jack Danforth of Missouri and Paul Laxalt of Nevada. But only Ford, Baker or Bush can simultaneously broaden Reagan's appeal to the moderates and offset his lack of experience. By picking one of them, he shows strong common sense, and a feel for quality. By picking anybody else, Reagan becomes just what Carter wants him to be -- the issue.