THIS STILL ISN’T AN ‘IDEOLOGICAL PURIFICATION’…. Former Bush speechwriter Marc Thiessen, now a Washington Post columnist, takes a look at yesterday’s primary results, and finds that his suspicions have been confirmed.

Last night two centrist Democratic incumbents failed to stave off challenges from the left in Democratic Senate primaries. Pennsylvania Sen. Arlen Specter was defeated by left-wing challenger Rep. Joe Sestak. And Arkansas Sen. Blanche Lincoln was forced into a runoff by her left-wing challenger Gov. Bill Halter [sic].

But don’t hold your breath waiting for commentators to decry these shameful efforts at the ideological purification of the Democratic party. When Sen. Bob Bennett is challenged from the right, it is an ideological purge. But when centrists like Specter and Lincoln are challenged from the left, it’s democracy in action.

This is consistent with the victim mindset Thiessen has been talking up in recent weeks — Republicans are facing criticism for purging those who stray from a far-right line, but Dems face no heat at all for primaries against some of their own.

Thiessen’s assumptions are badly flawed, leading to conclusions that don’t make a lot of sense.

When Bennett was rebuked by his own party, it was evidence of a party that’s shifted in an extreme direction — he has, after all, been a reliable conservative voice, who was this year deemed not quite right-wing enough. It’s a textbook example of an ideological purge — Bennett has been a rigid, conservative ideologue for years, but the GOP base decided they wanted someone even less open-minded.

The parallels to the Arkansas and Pennsylvania races are practically non-existent. In the former, for example, Thiessen’s comparison would only make sense if Blanche Lincoln were a liberal senator deemed insufficiently left-wing by the party base. But that’s foolish; Lincoln has proven herself to be a disappointment to most Democrats, not because of a handful of isolated votes, but because of her departures from party priorities on a wide range of issues, over the course of several years. Besides, charactering Lt. Gov. Bill Halter as a “left-wing challenger” is pretty silly given his issue positions.

In Pennsylvania, Specter and Sestak were largely on the same point on the political spectrum, giving primary voters a choice that had less to do with ideology, and more to do with Specter’s 30-year career as a member of the other political party.

If anything, the message from yesterday’s results had less to do with incumbency and the establishment, and more to do with Democrats backing candidates more in line with the Democratic mainstream.

Thiessen is understandably anxious to make it seem as if both parties are engaged in the same “ideological purification” efforts, but it still doesn’t have a basis in reality. After Crist, Specter, Scozzafava, Bennett, Grayson, and possibly even McCain, we see a Republican Party that’s effectively put a sign on the RNC’s door: “Only hard-right ideologues need apply.”

If there’s evidence to suggest Democrats are taking related steps, it’s hiding well.

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.