Presidential candidates are a breed apart, often propelled by traits that have shaped their careers and have deep roots in personal histories. Often their greatest strength can turn at supernova speed into their greatest weakness. The exact qualities that set them apart from the field trip them up eventually over the long haul of a presidential campaign. It's Ted Cruz's ramrod devotion to principle-or, its flip side, an unyielding insistence on getting his way-that could propel him to the front ranks of Republican contenders for president or render him unelectable. In this series of eBooks, The Washington Post is exploring in-depth all these key characteristics of the leading presidential contenders, the very characteristics that could help make one of them the country's next commander in chief-or forever sink their presidential ambitions.