If conservatives want to argue that Barack Obama's been flip-flopping on Iraq, I'll disagree but I could see what they mean. Charles Krauthammer, however, can't seriously believe that Obama's been "assiduously obliterat[ing] all differences with McCain on national security and social issues" since the end of the primaries.
Consider such non-obscure points as John McCain is pro-life and has said he wants to appoint judges who will restrict abortion rights, whereas Barack Obama is pro-choice. John McCain favors an amendment to California's constitution that would take back gay and lesbian couples' newfound marriage rights whereas Barack Obama opposes such an amendment. Barack Obama opposes a permanent American military presence in Iraq whereas John McCain favors it. Barack Obama thinks torture is wrong even when the CIA does it, whereas John McCain thinks it's great for the CIA to torture people. Barack Obama favors good-faith high-level negotiations with Iran, whereas John McCain wants to "bomb bomb bomb, bomb bomb Iran." One could go on, but it hardly seems necessary -- the only question is why The Washington Post thinks it's a good idea to publish columns that are designed to mislead its audience rather than to inform its audience, or why they think customers would want to pay money for a publication that behaves that way.