PRIMARY COLORS…. Last week, a few Senate Democrats strongly hinted that they had no qualms about siding with Republicans, from time to time, on filibusters of key pieces of legislation. It came on the heels of a wide variety of caucus members announcing their “concerns” about a public option. And Gitmo. And a climate-change bill. And the rest of the administration’s agenda.

When it comes to party loyalty, the majority isn’t exactly a finely tuned machine. Jon Chait argues today that some primary challenges might help.

Republicans use the threat of primary challenges to keep their members in line. Democrats don’t. And they don’t for strategic reasons. The conventional view deems primary challenges counterproductive. When senators or members of Congress depart from the party’s agenda, the thinking goes, they’re maintaining an independent profile necessary to win reelection. If you drag them too far to the left, you’ll just lose the seat to a right-wing Republican. Better to safeguard Democrats who will support their party most of the time rather than risk electing a Republican who won’t do it ever.

The logic breaks down in two ways. First, some members move to the right for reasons that have nothing to do with self-protection. Maybe they’re catering to special interests rather than home-state public opinion. (Take the squeamishness of many Democrats over a public health care plan, which commands over 70 percent public approval but virulent opposition from the health care industry.) Or maybe they’re just more conservative than their constituents. (Take Feinstein, or Joe Lieberman.)

Second, while the party has an interest in protecting the popularity of its elected officials, it doesn’t have an unlimited interest. Suppose, for example, that the Democrats had a chance to pass historic health care and climate change legislation, but that doing so would make Evan Bayh 20 percent more likely to lose his reelection bid. I’d take that deal. Obama would take that deal. But I’m pretty sure Evan Bayh wouldn’t.

And that’s the point. The possibility of a primary challenge could balance out Bayh’s incentives, thus aligning them more with those of the national party. He could still maintain a moderate profile on many issues, but might not, say, threaten to filibuster the once-in-a-generation progressive agenda.

I tend to think these challenges can be looked at 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 there are plenty of others facing far different circumstances. Dianne Feinstein, for example, represents a solidly “blue” state. Evan Bayh not only represents a state Obama won, but isn’t even facing a Republican challenger.

For that matter, Matt Yglesias noted this morning that there are two Democratic incumbent senators facing primary challenges next year — Specter and Gillibrand — and wouldn’t you know it, they’re both being pretty loyal to the party and the White House agenda right now.

Pressure, it seems, can be pretty effective. Dems may want to think about applying more of it.

Steve Benen

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.