Democratic Sen. Hillary Rodham Clinton will embark on a widely anticipated campaign for the White House Saturday, a former first lady intent on becoming the nation's first female president. Clinton disclosed plans to form a presidential exploratory committee on her Web site. Clinton's announcement, coming days after Sen. Barack Obama shook up the 2008 race with his bid to become the first black to occupy the White House, establishes the most diverse political field. Clinton is considered the front-runner, with Obama and 2004 vice presidential nominee John Edwards as top contenders. New Mexico Gov. Bill Richardson, who would be the first Hispanic, intends to announce his plans on Sunday.