PRIMARY COLORS…. It’s always fascinating to see what politicians are willing to do before they face a primary challenger and after.

A moderate Democrat who had vowed to oppose any effort by party leaders to push a health care bill through the Senate with a simple majority vote is rethinking her position.

Sen. Blanche Lincoln said Tuesday that she wants to see what is in the companion bill before deciding.

It was just a couple of weeks ago when Lincoln said she’d oppose using reconciliation to pass a House-approved budget fix. It wasn’t necessarily problematic for the proposal — the whole point is to get 51 votes for the separate bill, not 60 — but Lincoln’s opposition to her party’s legislative strategy was another reminder of her shift to the right on Democrats’ top priorities.

And then Lt. Gov. Bill Halter announced he’d take Lincoln on in a Democratic primary. Wouldn’t you know it, Lincoln, who was sure the Dems’ health care strategy was a bad idea, is suddenly feeling more open minded.

The primary effect can be awfully influential, can’t it? We saw it with Arlen Specter in Pennsylvania, we’ve seen it with Michael Bennet in Colorado, and we may soon see it again with Lincoln in Arkansas.

Just to be clear, 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. For all I know, Nelson may actually like a primary opponent to help prove how much progressive don’t like him.

But for every Ben Nelson there are Democratic incumbents who might be more reliable if they had to work a little harder to impress Democratic voters. Keep an eye on just how much work Blanche Lincoln is willing to do on this front.

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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.