Missouri state representative-turned-aspiring congressman Russ Carnahan should have it made. His father, the late Democratic Gov. Mel Carnahan, cast a huge shadow over state politics for three decades. His mother, Jean Carnahan, most recently was a U.S. Senator. And since age 8, he's been canvassing precincts, stuffing envelopes and building a get-out-the-vote machine. Not so fast. Between now and the August 2004 Democratic primary, Carnahan, 44, must craft a public persona distinct from the man whose shoes he'd like to fill, retiring Rep. and presidential candidate Dick Gephardt (D-Mo.); piece together the same coalition of working-class Catholics and black city...