WASHINGTON—Republican presidential candidate Newt Gingrich called for replacing the leadership of Iran and said that could be accomplished within a year, adopting a more aggressive posture toward the U.S. adversary than advocated by the rest of the Republican field or by President Barack Obama. Mr. Gingrich's comments came at a televised candidates' debate Tuesday night at a moment when multiple polls showed him leading in the race for his party's nomination. The eight candidates offered sharp distinctions over a range of national-security issues, among them the proper speed of the U.S. troop withdrawal from Afghanistan and continued foreign aid to...