'WANTED: President of large enterprise. Extensive travel. Experience in international affairs and national-security matters essential. Background in domestic affairs useful but not required. Apply to the American people."That's the job description of the American presidency these days, as shaped by the confluence of George Bush's resume and a remarkable sequence of ground-shaking global events - in Eastern Europe, the Middle East, and now the Soviet Union. And it's a job description that few prominent Democrats can match. A commentator has said that Mr. Bush seems more comfortable as "president of the world" than as president of his own country. In normal times, that characterization would signal political vulnerability. But in a period of seismic global changes, Bush - with his penchant for literal and figurative globe-trotting - appears to many of his countrymen as well suited for his job. This fact, reflected in Bush's high approval ratings, accounts for the unwillingness of heavyweight Democratic leaders to take on the president next year. This is a frustrating political season for the Democrats, who believe that Bush's lethargic performance on domestic issues from the economy, the environment, and civil rights to education and health care should hang like a political albatross around his neck. Despite all this, however, Bush appears to many Democrats as virtually invincible in '92. Foreign affairs will not always figure so largely in the electorate's political calculations as they do now; that pendulum swings, too. But, as they have been for the past half century, Americans will always be aware in presidential elections that they are voting at least potentially for the "president of the world." If they are once again to be competitive in races for the White House, the Democrats must somehow regain public confidence that their candidates and their party can be trusted with national and even global leadership in times of crisis. With their party largely shut out of the nation's foreign policy and national-security apparatus, it's not easy for Democratic hopefuls to gain Bush-like experience they can hold up before the electorate. But Democrats need more than just individual "experience." As a party, they still must fashion a coherent philosophy about the uses of American power and prestige in the furtherance of enlightened national and, to the extent possible, global interests. Multilateralism - the current lodestar for many Democratic strategic thinkers - will play an important role in future projections of American power in the world. But the United States, as the sole remaining superpower and the world's leading exemplar of political freedom, will still have to shoulder responsibilities only it can bear. Until the Democrats can speak convincingly to the electorate on this point, their access to the Oval Office likely will continue to be by invitation only.