The Democratic Party has reversed the ancient adage. In its ''reformed'' state, it's become a new dog that can't be taught an old trick -- how to win presidential elections. And relearning that trick could be the key to its survival.

Since Harry Truman left the White House 34 years ago, Democrats have won the Oval Office for only 12 years in all. In the same period, we've had Republican presidents for twice as long -- Eisenhower for eight years, Nixon and Ford for eight and, soon, Ronald Reagan for another eight. Jimmy Carter, who won one-third of the 12 Democratic years as a dark horse, almost lost to Gerald Ford and probably would have if the 1976 campaign had been a few days longer.

These gloomy musings -- gloomy for those who like competitive parties -- are inspired by Sam Nunn's saddening decision not to seek the Democratic nomination. And also by what was said about it. No sooner had the senator from Georgia bowed out than the party's soothing syrup pushers fell to assuring one another that his departure is a blessing in disguise.

After all, they say, Nunn is too far outside the ''mainstream'' of Democratic beliefs (as defined, presumably, by the single-minded activists). So it's just as well he saved his time and the party's.

The soothing-syrup merchants have a point. It wouldn't have been easy these days for someone like Sam Nunn to be nominated, notwithstanding the widespread belief that he (or someone representing the same wing of the party) would be the most effective candidate in November. The gauntlet of nonsense that people must run to be nominated by the Democrats these days is a forbidding prospect for any sane person.

Apart from proven abilities that make him a weighty presence in the Senate, Nunn's appeal was clear -- clear, anyway, to Democrats who relish the quaint pleasure of winning. With a sound vice presidential choice and some self-restraint by the party crazies, you could imagine Nunn stitching together sufficient electoral votes to win.

Begin with the South. No Democrat has ever won the presidency without southern electoral votes. Given the Republican dominance of the West, it seems unlikely one ever will. Barring some act of Republican self-immolation, the Democratic candidates rudely known as the ''seven dwarfs'' now constitute the circle from which the party must probably choose.

Albert Gore, a part-time Tennessean and a chip off a high-quality senatorial block, could have some appeal to the South. His southern credentials, however shaky, would at least be noticed. But as has been observed, presidential candidates ought to have a few wrinkles, and Gore is very young.

The supposed diminutiveness of the ''seven dwarfs'' is not in them but in their stars. They must compete in a lunatic nominating process, admitted by all to be crazy and counterproductive but resistant to repair. In its emphasis on the meretricious, the primary/caucus system is capable of making Solomon himself look a fool, and a nauseatingly obsequious one.

It is vastly overinfluenced by single-issue and special-interest manias. The influence of elected officials or party organizations -- coalition-builders by nature -- is negligible by comparison with that of zealots for this or that cause: usually a cause having to do with values or life styles inaccessible to normal politics. And the zealots show about as much tolerance for political variety (witness the sneering at Nunn's ''incorrect'' views on contra aid) as, say, Augusto Pinochet of Chile.

What primary voters lust for mainly is the titillation of being agreed with and plied with zany promises. Oh, sure, we'll move the U.S. Embassy in Israel from Tel Aviv to Jerusalem! Put a homosexual on the Supreme Court! Establish bilingualism in California, Texas and Florida! Make Bella Abzug secretary of defense, Ralph Nader secretary of transportation and William Kuntsler attorney general. Anything you want; just ask.

A parody? Of course, but not far from the bizarre scenes being enacted in obscure corners of Iowa and New Hampshire.

Sam Nunn might not have made it. But he is the sort of candidate who reminds us that the Democratic Party once won elections and produced notable presidents; indeed, that it is the oldest vehicle of political liberty in the world.

But in their new phase, to resume the metaphor, the Democrats bark more and bite less.