There’s another analogy, however, that should worry Democrats even more: Not between General MacArthur and General McChrystal, but between General Dwight Eisenhower and General David Petraeus. Pundits have mused about the Eisenhower-Petraeus comparison before, but the Afghanistan slugfest gives it new relevance. In the late Truman years, MacArthur, Joseph McCarthy, and the rest of the Republican right wing were a bit like Sarah Palin and Glenn Beck today. They succeeded in bloodying the Democrats and scaring the country about overseas threats. But their overseas warmongering and domestic radicalism made them too extreme to ever win national office themselves.

Ike was different. He exploited the right’s hysteria, and yet sailed above it at the same time. He refused to condemn McCarthy, and implied that he too believed that Truman’s containment policies constituted appeasement, but he maintained his calm, soothing tone. As a war hero who stood apart from the partisan brawling around him, he retained a personal brand far stronger than either party’s…

All this is wildly speculative, of course. But there’s a political logic to it: Parties that have grown narrow and extreme tend to spiral downward until they nominate someone who is not beholden to their narrow, extreme base. That person has to be so popular that he or she can defy the normal rules about how candidates get nominated. Right now, David Petraeus is the only Republican who fits the bill. In the weeks ahead, McChrystal may become a conservative folk hero for opposing Obama on Afghanistan. But for Democrats looking toward 2012 and 2016, it’s Petraeus who represents the real threat.