“Rick Santorum was a sitting senator who, in re-election, lost by 19 points,” Donald Trump, a Mitt Romney supporter, said recently. “Then he goes out and says, ‘Oh, OK, I just lost by the biggest margin in history, now I’m going to run for president.’ Tell me, how does that work?”

Trump exaggerated only slightly; Santorum actually lost by 18 points. But there’s no doubt the most glaring weakness in the case for Santorum’s electability is that 2006 re-election loss. After two terms in the Senate, the voters of Pennsylvania simply threw Santorum out on his ear.

Why? Santorum explains it mostly by saying ’06 was a terrible year for Republicans. Indeed, the GOP, in the sixth year of George W. Bush’s time in office, did lose control of both the House and Senate. But why did Santorum lose so badly?