One way to evaluate these competing explanations is to ask: If this is true, what else must be true? If the “Cornhusker Kickback” did in fact repel voters, we’d expect to see support for health care plummet after mid-December, when the deal was struck. But in fact, as John Sides notes, opposition to reform, which had been rising steadily throughout 2009, actually softened in national polls at about that time. (The more relevant information for interpreting the election would be Massachusetts-specific, but I’m not aware of comparable data there.) The Nebraska deal certainly gave opponents of reform more reasons to oppose it. It may also have energized them, which could in turn have had electoral consequences. And, in fairness, it’s possible that it changed minds. But the data proving that point, despite Tumulty’s effort to link her reporting experience to the Post poll, is in short supply.

There are two frustrations here. While there is reason to be skeptical about the Cornhusker claim, we’re not left with much in its stead. The alternative explanation doesn’t say why Brown drew as many votes in Massachusetts as John McCain did, while the Democratic vote collapsed. (A tentative attempt to do so is here.)

The bigger problem, though, is that we’re not left with much guidance to give journalists. Tumulty, after all, did exactly what we’re always exhorting the press to do: she got out of the Beltway bubble and talked to voters on the ground. But if the language voters use to talk about their choices is lifted from the Beltway conversation, even as local factors continue to play a role in determining elections, the value of that approach is less clear.

While we search for a way to crack this nut, the best response for journalists is not to throw their hands up. We do need sharp reporters like Tumulty out there talking to voters, seeing which messages get picked up, and searching for those nuggets that don’t come from a list of talking points. At the same time, though, the press should be skeptical of easy explanations—and humble about its ability to figure out why things happen in the world of politics.

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Greg Marx is an adjunct lecturer at The Medill School and a facilitator with The OpEd Project. She served as an editorial board member, columnist, library director, and No. 2 in the features department of the Chicago Sun-Times.