As anyone who’s read my writing can probably tell, I think political journalism should pay more attention to what political scientists have to say. So I was heartened to see that today’s New York Times includes an op-ed co-authored by Andrew Gelman, the Columbia statistician and political scientist, along with Nate Silver of FiveThirtyEight.com and Columbia research associate Daniel Lee. And they make a good case for their argument, which is that:

when it comes to politics, “ObamaCare” could hardly be more apt: lawmakers’ support for or opposition to reform generally has less to do with the views of their constituents and more to do with the issue of presidential popularity.

In other words, one factor influencing senators’ thinking will be (rightly or wrongly) Barack Obama’s popularity in their home state. In particular, Obama’s ratings will be more important than local opinion about health care itself:

But in general, senators seems to be less interested in what their constituents, old and young, rich and poor, might think about health care, and more interested in how they feel about President Obama.

Interesting stuff. But, while we’re weighing which stripe of public opinion matters more, it’s worth noting that the public’s views—on any topic—are not the only thing shaping Congress’s approach to policy.

Consider, for example, the several instances in which two senators hail from the same party and the same state (and thus are dealing with the same public), but have staked out different positions on health care. In Virginia, Jim Webb is counted by the authors among the “49 Democrats who fully support the bill,” while Mark Warner “supports with reservations.” The same is true in Montana, where Max Baucus’s reservations are not shared by Jon Tester, and North Dakota, where Byron Dorgan is more bullish than Kent Conrad. (Delaware, where the fully-supportive Ted Kaufman is a stand-in for his long-time boss Joe Biden, and Connecticut, where Joe Lieberman caucuses with Democrats but is an independent, seem like special cases.) On the other side of the partisan aisle, of Maine’s two Republican senators, Olympia Snowe has offered signs of support for reform, while Sue Collins seems staunchly opposed (she is counted by the authors as “opposes in practice”).

The variation suggests that there’s more shaping senators’ thinking—and their likely votes—than sensitivity to public opinion. This is a point that Gelman himself recently made at The Monkey Cage:

Congressmembers and Senators can pretty much vote how they want on most issues, whatever their constituents happen to believe. Not always, of course, but a representative can take a much more liberal or conservative line than the voters in his or her district or state, and still do fine when election time comes.

I asked Gelman about this today via email, and he replied that “a combination of partisan considerations, and the influence of interest groups, and their own considered opinions on the issue” all influence senators’ thinking. Of the op-ed, he said, “Certainly we didn’t mean to imply that Obama’s popularity in the state was the only factor. The real point is that senators are not really directly responding to their constituents’ views on health care, or at least it doesn’t look that way based on our data.”

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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.