It is again that time in a presidential year when the fastidious votaries of liberal purity rediscover reasons for refusing to support the nominee of the party they usually call home.

It is not, they echo the Dixiecrats of old, that they are leaving the party, but that the party's leader has left them, left the old Democratic verities and abandoned the compassion and courage and commitment that marked the leaders of the glory days.

Besides, they argue with considerable heat, it is not as though the election of the Republican candidate would make any real difference, considering the policies of the Democratic standard bearer. He, after all and depending on the year, has deserted 1) The New Deal. 2) The Fair Deal. 3) the New Frontier. 4) The party of Roosevelt-Truman-Kennedy and- Humphrey. 5) The principles of George McGovern. 6) The principles of the minority of the party that opposed him.

It is an old refrain, ghosting forward from past autumns, and in remarkable fashion some of the same persons have been singing variations on the melody for over three decades. s

They were, first the valiant guardians of Franklin D. Roosevelt's heritage who tried to convince the Democrats of 1948 that FDR's handpicked choice for vice presdient should not be allowed to sully his memory by remaining president. Harry Truman was a traitor to the New Deal and nothing but a pretty politician, they sang in unison. What the party and the nation deserved was someone different and better. Someone like Gen. Eisenhower, whatever his politics might be. Or that failing, Justice Douglas. But not, oh God of progressive orthodoxy, that miserable little nothing, Harry Truman.

Ah, but they were given the sanctity of thought and deed they sought with John F. Kennedy, having meanwhile gladly followed the leadership of the right sort of Democrat into two straight landslide seasons, glad to talk like a liberal after a political career of careful compromise. He let some of them not only sniff the hem of power but actually drape themselves in its glory, and his record was placed in the Pantheon of political greatness forever.

Dismal days thereafter. Lyndon John provided the ultimate effrontery, which was to put onto the statute books what Kennedy had failed to achieve. But they were redeemed, because LBJ prosecuted the war in which Kennedy had believed so passionately with the same passion, while theirs had wanted with the war's fortunes. Lyndon could be hated as the usurper he was, and dumping the Democratic incumbent was suddenly back in fashion after less than two decades of respite.

It was not enough simply to dump the president however. Hubert Humphrey, who had carried the liberal standard through the corridors of Congress while others were lecturing on its demise, was deemed contaminated forever because he shared the dirty little open secret of virtually every Democratic officeholder of the time: he had supported the war -- and thus a vote for Hubert was a vote for war. Let's sit this one out, Nixon or no Nixon.

Ideological purity is a catching disease, however, and many Democrats were infected with the virus when that perfect ipitome of liberal-left rectitude, George McGovern, was nominated in 1972. By the next time around, the Nixon Presidency had produced at least one positive effect. He had convinced many Americans, and virtually all Democrats, that there could be something worse than an imperfectly liberal nominee.

It has not proved to be a long-lived lesson. The nation's journals are full of the trendy deserters' favorite refrain yet again. It's time to come home, America, home to John Anderson, the Republican from Illinois. What are a few quirks in his past compared with the warts of Jimmy Carter's present? That Anderson is a man of opaque views and mixed votes on many issues is as nothing when placed against his overwhelming qualification: he is not Jimmy Carter, the current president of the United States.

He is also not going to be the next president of the United States, but that is deemed beside the point. What matters is teaching a lesson to that defector from Democratic Party values who won more Democratic Party primary votes than any other man in history. What matters is to reestablish that a tired, aging and finally irrelevant remainder of a great era can play spoiler. Who knows what access to power might be available in the next Democratic presidency (if it only hurries along) once it is understood that they can deny the job to those they despise, no matter what a majority of Democratic voters might want?

And what of the real alternative to Jimmy Carter? Why, Ronald Reagan isn't really Ronald Reagan, and it doesn't matter if he is. He won't actually adopt policies that are much different from Carter's.

He will obviously be willing to deal as fairly with small nations -- as with the Panama Canal treaties -- as with large. He will, of course, be as anxious to negotiate the next small step toward the meaningful control of strategic nuclear weapons as Carter. He will most assuredly stand forth as firmly for human rights in those right-wing torture chambers that call themselves our friends as he will be in speaking to that huge totalitarian monolith that knows it is our enemy. Nor will he name federal judges any less representatives of all Americans or any less concerned about the extension of full rights to all than Carter.

So it is another sunny fall for these paragons of Democratic Party virtue. They can put aside the mundane business of making real choices in the real world and call for the restoration of the perfection that never was under the leadership they never possessed. Harry and Lyndon and Hubert must all be laughing a little wryly as they watch the ritual madness of their old adversaries from afar. They know that these defenders of the faith do finally have one significant gift, much akin to turning lead into gold. They can help guarantee that the majority party of the past becomes the minority party of the future, complete with principles that have relevance to at least 10 percent of the American people.