COLUMBIA, S.C. — A week after leaving his post as U.S. ambassador to China, Jon Huntsman appeared to edge closer to challenging his former boss for the White House, meeting with top Republican Party leaders here and telling an audience of graduates that America is “still the envy of the world, we are still as full of potential as ever.” “I know there are many in China who think their time has come, that America’s best days are over, and there are probably some in this country who have lost confidence and think that China is the next best thing,” he said.