PRIMARY COLORS, REDUX…. A couple of weeks ago, Jon Chait had a great piece arguing that the Democratic majority on the Hill would have fewer problems with party discipline, and more success on the party’s agenda, if more vacillating members faced primary challenges.


Nate Silver reinforced the argument over the weekend, noting the trend in Sen. Arlen Specter’s voting record. The first column shows his willingness to vote with the Democratic majority when he was a Republican, facing a likely GOP challenger from the right. The middle column shows Specter voting with Dems after his party switch, but before he had to worry about a Democratic primary opponent. And third is after Rep. Joe Sestak said he planned to get into the race.

A little pressure, in other words, can go a long way. Specter went from being a moderate Republican to, eventually, “behaving like a mainline, liberal Democrat,” at least in part because he has to worry about impressing Democratic primary voters in a “blue” state.

Would Specter be as dependable on the party’s agenda if Sestak weren’t part of the equation? Silver makes the argument that he wouldn’t, and I’m very much inclined to agree.

I still think these challenges can and should be considered on a case-by-case basis. It’s tough, for example, to threaten Sen. Ben Nelson with a primary challenge from the left. He represents a pretty “red” state (Nebraska), and for all I know, Nelson may actually like a primary opponent to help prove that he’s not part of the Democratic mainstream.

But for every Ben Nelson there are a few Democratic incumbents — Dianne Feinstein, Evan Bayh, I’m looking in your direction — who might be more reliable if they had to work a little harder to impress Democratic voters.

I should add, by the way, that this dynamic is also playing out on the other side of the aisle. Matt Yglesias had a good item over the weekend, noting Sen. Chuck Grassley (R) with his eye on re-election next year. No credible Democrats have yet stepped up to launch a serious challenge, but Grassley is worried about a far-right opponent in a Republican primary. “Which means,” Matt noted, “that the only thing Grassley has to do to secure his tenure in office is obstruct health care reform.”

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.