WHAT A DIFFERENCE A PRIMARY MAKES…. This comes up from time to time, but it’s almost entertaining to see the kind of effect a Democratic primary is having on Arlen Specter.

Sen. Arlen Specter (D-Penn.), who’s looked at POTUS from both sides now, was just on the Bill Press radio show suggesting the South Carolina firebrand Joe Wilson be censured for his heckle heard round the world.

Said the Republican-turned-Dem:

“He apologized immediately afterward but I don’t think that’s adequate… If an apology is the consequence of an outburst I think we can expect more — that’s not a sufficient penalty that’s not a sufficient price to pay… I’m not saying the guy should be kicked out of the House… But there ought to be some rebuke, reprimand, censure — something that will discourage that kind of conduct in the future. If you do that to the President, it’s open season.”

This comes a day after Specter announced that he would tell President Obama “emphatically” that the U.S. “needs the public option.”

Specter initially opposed the public option, even after switching to the Democratic Party, but changed his mind after Rep. Joe Sestak (D) announced he’d challenge Specter in a primary next year.

Realistically, is there any way in the world Specter would be demanding a public option and demanding punishment for Joe Wilson if he didn’t have to impress Democratic primary voters? It’s hard to imagine.

This also reinforces something Nate Silver noticed in July — before Sestak launched a primary challenge, Democratic Specter voted with his party 69% of the time. After Sestak announced, Specter’s party loyalty improved to 97%.

A little pressure, in other words, can go a long way.

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. 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. It’s certainly doing wonders for Specter.

Our ideas can save democracy... But we need your help! Donate Now!

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.