After 16 years, the Clinton era may be coming to an end, presenting Democrats with a historic but potentially wrenching transition and a challenge to Senator Barack Obama as he seeks to reconcile a deeply divided party. Bill and Hillary Rodham Clinton have been at the heart of the Democratic Party since Mr. Clinton steered it back to the White House in 1992, with a campaign that combined a moderate appeal with the hard-edged political tactics that had once been the province of Republicans. Mrs. Clinton seemed poised last year to lead Democrats into the general election campaign if not...