Heeding a trumpet that sometimes calls retreat, Alan Cranston's campaign operatives recently went on a weekend retreat to almost- heaven West Virginia to decide what to do next. They should have come to Chevy Chase, Md., and asked me.

Cranston should announce that, starting now, his running mate will be Rep. Parren Mitchell of Maryland. (Actually, this idea comes from a Democratic professional who has the instincts of a saboteur, including the instinct for anonymity.)

Let us assume Mitchell would be willing. He is a serious, even professorial (he is a lapsed professor), seven-term legislator. He practices Cranstonesque liberalism. And he is black. If Cranston can attract the lion's share of the severe liberals who, because of their intensity, are disproportionately important in the nominating process, and if he can add a large number of black voters, he may win large enough pluralities in early primaries, and then, who knows?

The country may not be ready on Nov. 6, 1984, for a Cranston-Mitchell ticket. But since when is it required, or even cricket, to think of the general election when scrambling for the nomination?

Would the choice of a running mate a year before the convention be considered an act of desperation? Perhaps. Ronald Reagan's choice of Richard Schweiker before the 1976 convention was an act of desperation. And it did not work: it did not pull Pennsylvania delegates and others to Reagan. But Cranston, like most of his competitors, will be desperate soon enough. Besides, the act could be coated with high principle: the voters in primaries deserve to know as much as possible about what they might end up stuck with.

Daydreaming about potential running mates is a useful exercise for illuminating the strengths and problems of particular presidential candidates. For example, Walter Mondale should choose Lee Iacocca.

Mondale has what can perhaps be called Norwegian charisma. It is, no doubt, stirring to other Americans of Norwegian descent, but there are not enough of them. Iacocca is the Rice Krispies of corporate America, full of snap, crackle and pop. Or, as a Democratic professional says, Mondale is like Chinese food: he needs MSG (monosodium glutamate) to give him punch. Iacocca is MSG.

Mondale has acquired a reputation as a candidate who is too cautious by half. Choosing Iacocca would show a capacity for surprise and risk-taking. Furthermore, as keeper of the flame of Hubert Humphrey's politics, Mondale is in danger of being perceived as the Last Liberal, someone who has sweet intentions but no knack for turning them into policy successes. Well, think of federal domestic programs as a kind of collective Chrysler Corporation, circa 1979. . . .

I hear Democrats say that John Glenn should choose New York's governor, Mario Cuomo. Cuomo, they say, would cure Glenn's Midwestitis, which is a form of ethnicity deficiency. Besides, Italian-Americans are increasingly important, from coast to coast. One runs Yale and, even more impressive, one manages the Los Angeles Dodgers. (The Dodgers are doing better than Yale but not as well as Chrysler, which is run by. . . .)

Many Italian-Americans are Republicans. Many blue-collar Catholic Democrats have been voting Republican regularly. That is one reason why New Jersey has gone Republican in the last four presidential elections. Many Democrats who see New Jersey as a symbol of their party's problems say something must be done and that Cuomo as vice president is the something.

On the other hand, there are those who say that if Democrats cannot count on most of their ethnic base and most of the industrial Northeast, they are in terrible trouble, given their problems in the South and West. Because of those problems, the presidential nominating process is an ordeal to determine who shall have the pleasure of picking as his running mate Sen. Lloyd Bentsen (D-Tex.).

One complaint about Bentsen is (in the words of one Democrat) that he probably had white upholstery in his living room even when the kids were young. That is, he is too polished and well-tailored to cause Democratic pulses to race. But what will get Democrats energized is the idea that Reagan wants nuclear war in order to blow up the Social Security System.

Besides, Glenn does not believe in ideologically "balanced"--that is, mixed--tickets. Such a ticket means that a heart attack can frustrate an electoral mandate. Bentsen is more like Glenn than Cuomo is. If Bentsen's record and manner are somewhat businesslike, well, the Democratic Party's problem has not recently been that it seems too sober when handling the public purse.

And two more things. No Republican has won the presidency without carrying Ohio. And no Democrat has won without carrying Texas.