TARA LIPINSKI was so at ease last week in Nagano, Japan, that a number of insiders were marking her as the favorite for the Ladies Figure Skating gold medal, even though MICHELLE KWAN has bragging rights as U.S. National champion. The two rivals, together with another former American champ, NICOLE BOBEK, are part of the team that has the Olympics murmuring, "U.S. medals sweep." That kind of talk isn't, of course, going down well with other competitors. Russia's MARIA BUTYRSKAYA settled on her target a month ago, sniffing, "Lipinski made mistakes in many competitions this season. I am a good skater too." Last week at practice, Butyrskaya looked impressive. Among the Americans, however, it is Bobek who is vulnerable--but not just because Lipinski and Kwan are considered by most other skaters to be in a class of their own. "I think we'll win the gold and silver," says Rosalynn Sumners, the 1984 Olympic silver medalist. But, she adds, "I think a sweep will be tough." Why? "Politics."