Mitt Romney is more than holding his own with Barack Obama tonight. Only two other challengers have done as well debating foreign policy with an incumbent president—Ronald Reagan against Jimmy Carter in 1980 and, to a lesser degree, Bill Clinton against George H.W. Bush in 1992. Reagan and Clinton won. Romney is now on track to becoming the third challenger to win in the last 32 years—and the first in 80 years to defeat an incumbent who didn't have a primary challenge. Tonight, Romney seems as fully capable as—probably more capable than—Barack Obama of being the next president. He probably...