With less than a month to go before the scheduled showdown House vote on the North Atlantic Free Trade Agreement, the Clinton administration is learning that it will have to pay heed to its Republican supporters as well as to wavering Democrats. About time. Free trade is identified with the GOP at this moment in history, but so is opposition to higher federal taxes and fees. Last week, in another legislative fumble, the White House offered and then quickly backed away from a proposal to double federal levies on all international airline passengers, trucks and rail cars coming into this country. The move was intended to offset a $2.4 billion loss in tariff revenues over the next five years as NAFTA lowers trade barriers. But it triggered a protest from 27 House Republicans whose votes are needed to offset the sizable protectionist tide in Democratic ranks. The quick administration retreat toward a compromise on this front was matched by its prompt opposition to a move by some Senate Democrats to reimpose tough, automatic trade penalties against nations the United States unilaterally judges in violation of international norms. This is the so-called Super 301 provision which was adopted for a couple of years in the late Eighties and then prudently junked. If the Super 301 challenge to international trade rules were resurrected, it would doom the frantic push in Geneva for a new worldwide General Agreement on Tariffs and Trade before a scheduled Dec. 15 deadline. President Clinton deserves high marks for standing tall. It should demonstrate to skeptical Republicans that he is a genuine free-trader, a Democratic leader willing to take on big labor and those parts of the environmental movement that have gone protectionist. Mr. Clinton, however, has not yet displayed the kind of top-priority, full-court-press attention that will be necessary if NAFTA is to win congressional approval. Although much of the debate has centered on economic issues (Will NAFTA create or lose jobs? Can the government really provide meaningful retraining for displaced workers? Which regions and industries will benefit or suffer?) the proposed binding together of the Canadian, Mexican and American markets is of far greater political and foreign-policy significance. It is regarded throughout Latin America as a test of U.S. goodwill toward its hemispheric neighbors. It is considered worldwide as a measure of how much other nations can rely on the United States to ratify agreements negotiated in good faith. For half a century, this country has encouraged global economic growth by opening up commerce among nations. To preserve this policy, the Clinton administration now has to show it can secure the adoption of NAFTA and bring about a successful end to GATT negotiations. The next few weeks are crucial. They will test Mr. Clinton's acumen as president and as a Democrat capable of leading his party away from its protectionist drift.