STILL infant, the l992 presidential campaign in one regard already resembles its recent predecessors: It proceeds by inflating one electoral myth after another, much as a street vendor pumps helium into a child's balloon. It then watches with fascination as the myth swells; and when it explodes, it goes on to another without pause or apology.The latest of these curious political trial balloons is the notion that what really matters to Americans are domestic matters, not foreign affairs, and that the president's domination of the latter is actually, mirabile dictu, a disadvantage. The public, we are told, faults George Bush for spending too much time on events abroad. An innocent observer might be excused for thinking that this notion is too neatly self-serving - for Democrats frustrated by the acclaim the Republican president has received for his handling of the Gulf war and relations with the former Soviet Union. But no, even a nervous White House rushes to inflate the myth, by hurriedly postponing the president's trip to the Far East. Of all the pumped up, hyped up, silly ideas floated in recent campaigns, the argument that foreign affairs takes a back seat to domestic is easily the most ludicrous from a historical perspective. For in fact, foreign policy and national security issues have been dominant, or at the least co-equal, in every United States presidential election from Franklin Roosevelt's third-term vi ctory in 1940 on to the present. Where is the exception? Certainly not the 1952 contest, when Dwight Eisenhower, who had made his mark leading the allied armies to victory in Europe, capped his campaign by promising to conclude an Asian war: If elected, he said, "I shall go to Korea." Neither was it 1960, when the big issue was an alleged Republican-caused "missile gap," and when the newly elected John Kennedy, in an inaugural speech dominated by foreign policy, promised that the US would "pay any price, bear any burden, meet any hardship, support any friend, oppose any foe to assure the survival and the success of liberty" around the world. The Vietnam-war-centered elections of l964, 1968, and 1972 were no exceptions, either. And the idea that it's really domestic issues that matter didn't take root in the 1980 election. The campaign of incumbent Jimmy Carter scored its most telling blows by charging that Ronald Reagan would be rash, even "trigger happy" in commanding the world's most powerful military. Mr. Carter's ultimate defeat accrued, more than anything else, from the public's conclusion that he lacked the capacity to represent America and her idea with sufficient firmness and clarity in world affairs. In one sense, of course, things closest to home are typically seen as the most important. Except in the midst of major wars, when national survival or critical US interests are immediately engaged, Americans will not accord to troubles abroad the same importance they do to problems in their own cities and towns. This doesn't mean, however, that foreign policy plays a secondary role in national elections. For one thing, while Americans wish that the world were somehow more obliging and that the country co uld promote its vital international interests with less cost, they don't think that it can. Foreign and national-security affairs also dominate presidential electioneering because - though less imposing when set against the sum total of domestic concerns - they occupy the very center of things a president is able to shape and for which he is held accountable. The status of the US economy is of immense importance to Americans. But, while majorities now say they disapprove of President Bush's handling of the economy, only 28 percent (according to a Gallup poll for Newsweek taken Oct. 24 to 25) "think a Democratic president would handle economic conditions better," and more blame Congress for the current state of affairs. Americans understand that the president is but one actor among many in domestic policy. He is bound by all the checks that inhere to the separ ation of powers and federalism. Influential interest groups engulf every aspect of domestic decisionmaking. In foreign affairs, though, the president dominates the scene, and national success hinges on what he does. Perhaps most important of all, US leadership in foreign affairs has a direct and powerful emotional link to American nationalism. "Americans like to think their nation is No.1 in the world," columnist Tom Wicker observed recently. "Whatever else they may think of their president, they expect him, as their representative, to be a formidable figure on the world stage." In the 1992 campaign, Bush will likely derive great strength domestically from his successes internationally.