It is customary in American political campaigns for the Opposition Party to blame the Party in Power for everything then currently yukky. Thus any increase in devil worship, drought or the divorce rate or any decrease in the dollar, organ donors or reading scores is laid squarely at the door of the Ins by the Outs. In the 1988 presidential campaign, the Democrats have been faithfully observing this political tradition.

But so far in the 1988 Outs message there has been an even more established cultural tradition and national character trait that has not been observed, and that is optimism. From 1932 to 1968, an era during which the Democrats clearly represented optimism and pragmatic change to a big majority of American voters, only one Republican won the White House. The numbers since then have not been nearly so impressive.

Optimism was an American virtue even before there was an America. It took optimism to cross the Atlantic. It took optimism to leave the known for the unknown. From the beginning, Americans were gamblers and they were pioneers. And the settlers quickly became rooters, trumpeting the virtues and advantages of their new home town, because growth meant stability. Loss of popularity meant loss of population and with it the loss of any possible security for the settler. There were few advocates of ''no-growth policies'' among the early Americans, and pessimism was a stab in the back to the community.

Now, think of Democratic liberals from 1945 to 1965: FDR, Harry Truman, Jack Kennedy, Hubert Humphrey. Remember the pictures of these men? They were smiling. Roosevelt's jauntiness, Kennedy's grin, Humphrey's resilience. Remember their words? All these men were confident and optimistic about what we could do together. They all believed that government is an instrument with which to do things, to improve, to help make ours a world in which the strong are more just and the weak are more secure.

It was that liberal leadership, confident and optimistic, that led the nation away from the snug comfort of American isolationism and later disproved the foreign fears about the prospect of American imperialism. And it was the success of liberal programs, according to Martin Anderson, President Reagan's first domestic policy adviser, that cut the percentage of Americans living in poverty by more than half between 1960 and 1980. In the last year of Dwight Eisenhower's second term, nearly one out of five Americans was living in poverty. By the last year of Jimmy Carter's only term, that figure had dropped to one American in 15.

Somewhere between Watts and Saigon, liberals and Democrats lost their confidence, their optimism and their way. Ignoring the weight of evidence and the personal experience of their fellow citizens, many Democrats and liberals chose to deny that Americans had become, as a direct consequence of liberal programs, healthier, better fed, better housed, better educated and residents of a cleaner and more wholesome physical environment. Instead, who celebrated an improved America to his own political advantage? You got it -- the fellow who had opposed all those intrusive, expensive federal programs such as Medicare, the civil rights acts and the environmental laws: Ronald Reagan.

Optimism was not only vital to build our nation; it has been warranted by our history. Today, optimism remains key to successful political leadership. We still prize leaders who can tell us practically and confidently where they would lead us. And confidence is a first cousin of optimism.

For the 1988 Democratic candidates to blame Reagan and the Republicans for the latest outbreak of static cling or gout is fair and expected. But to capture the imagination (and the votes) of the electorate, Democrats would do well to remember the music of our politics as well as the words by reflecting on an anecdote told about their fellow party member, Andrew Jackson. When Old Hickory was being buried, one of his slaves was asked if he thought that Jackson, an irregular church-goer, would go to heaven. Back came the answer about that great Democrat and great American: ''He will if he wants to.''