THE BARACK OBAMA who mounted the podium last night for his first State of the Union address is the same man who ascended to the presidency a year ago: even-tempered, intelligent, charismatic. Yet the political rough-and-tumble of his first 12 months in office -- especially the stunning defeat of his party's candidate in the Jan. 19 Massachusetts U.S. Senate election -- has cast his signature health-care initiative into limbo and diminished, at least temporarily, his once dominant stature. His challenge last night was to restore confidence and reassert leadership.