Rick Santorum, the dark-horse 2012 US presidential contender whose deeply conservative, often-gruff campaign put nominee Mitt Romney to the test, announced Wednesday he is launching a second run for the White House. It will be a more difficult proposition for him this time around. With 15 or more prominent Republican presidential contenders as his 2016 rivals, Santorum, 57, is even more the White House underdog than he was four years ago.