It’s not quite on a par with 9/11 truthers or Obama birthers, but recently a number of liberal commentators have descended into the fever swamps of denialism by rejecting the most basic facts about our debt and deficit. Mind you, they are not arguing about the best policies to reduce the debt — taxe hikes vs. spending cuts — but actually denying that the problem exists at all. Paul Krugman, for example, pronounces the debt problem “mostly solved.” Matt Yglesias of Slate asks, “What sovereign debt crisis? There certainly isn’t one in the United States.” Bruce Bartlett, every liberal economist’s...