THE defeat of former US Attorney General Richard Thornburgh by newcomer-incumbent Harris Wofford in Pennsylvania's senatorial race is proving a revelation for Democrats. They've learned how to exploit what always seemed a weak spot in George Bush's presidential armor - domestic policy.Last winter's showdown with Saddam Hussein in the Gulf war brought Mr. Bush the highest presidential ratings of modern times. He looked invulnerable all summer. Now, however, economic indicators are down and so is Bush's popularity. It's charged that Bush cares only about foreign policy and schmoozing with foreign leaders. Democratic hopefuls in New Hampshire talk of little else. Mr. Wofford in Pennsylvania, with a passion reserved for politicians in the final week of what had been thought a close campaign, said it was outrageous that Bush was "helping Turks, Kurds, Bangladeshis" and other presumably less deserving peoples rather than the "American worker." His call for national health care seemed particularly potent. Some Democratic leaders even suggested Bush should have stayed home last week from the US-hosted Middle East peace conference in Madrid. That's inane. What should Bush have done? Sent a note to the assembled Middle Eastern leaders that he had a pressing lunch engagement with Senate majority leader George Mitchell instead? Doubtless Bush will have to pay more attention to US matters; he's already canceled a trip to Asia. He can't say the economy is fine if it isn't. Bush's economic remedy - for Americans to go out and buy a new house and car - seems right out of the Eisenhower era. More leadership in the knotty problems of race, crime, education, the environment, health, and child care is needed. These are roll-your-sleeves-up issues. By comparison, phone calling to foreign leaders, jetting off to parts far flung, and toasting diplomats can seem to the public an elite game for self-important internationalists who are removed from the daily problems of ordinary people. Yet the question of foreign versus domestic policy is in danger of falling prey to demogoguery that plays to ignorance and isolationism. Foreign policy is important. The world is a fragile place today. Major structural changes are taking place in Europe, Africa, and Asia. Outcomes are hardly clear. The remaining superpower has significant responsibilities. Take the issue of aiding Kurds, for example. The American people, by a large majority, accepted the Gulf war. Such an operation makes demands. Responsibilities must be met. The US cannot simply turn its back on the world. Rhetoric about cutting overseas help may get votes, but it feeds narrowness, hiding from voters the complex realities that must be faced in an interdependent world. Good foreign policy is good domestic policy. The inverse is also true. A US in economic or cultural crisis will not help the rest of the world.