THE RELATIVE RATINGS OF ‘FAITHFUL CONSERVATIVES’…. Harold Meyerson explained in a column last year that bipartisan policymaking has been rendered largely impossible by the Republican Party’s shift to the hard right. Citing research from Jacob Hacker and Paul Pierson, Meyerson noted that a House Republican in 2003 had a voting record 73% more conservative than the median GOP member of the early ’70s.

And the party has moved quite a bit further since 2003.

As a result, the Republican “mainstream” is nowhere near its previous points on the ideological spectrum. Dana Milbank noted in a recent column, for example, that Republican Sens. Bob Bennett of Utah and Lisa Murkowski of Alaska had been purged in GOP primaries, and used the phrase “faithful conservative” to describe the latter.

This led a reader on the right to complain. Murkowski, the reader said, is actually “the most liberal … Republican Senator west of Maine,” which Milbank would see if he went through American Conservative Union ratings.

So, Milbank did just that, evaluating ACU scores for Republican lawmakers over the last four decades. He found that using “the purity standards” conservatives applied to oust Murkowski and Bennett would also have led to the purge of “many, if not most, of the leading Republican lawmakers of the past 40 years.”

Murkowski’s a lifetime ACU rating is 70.2%. Bennett’s is 83.6%. If these two had to go, so too would three former Senate Majority Leaders (Bob Dole, Howard Baker, and Hugh Scott) and three former Senate Whips (Alan Simpson, Ted Stevens, and Robert Griffin).

Among those past senators who would be vulnerable to a purge in today’s Republican Party: Nancy Kassebaum of Kansas (56 percent), Al D’Amato of New York (57 percent), Slade Gorton of Washington (70 percent), Mike DeWine of Ohio (79.8 percent), Gordon Smith of Oregon (68.8 percent), John Warner of Virginia (79.2 percent), Pete Domenici of New Mexico (74.1 percent), Ben Nighthorse Campbell of Colorado (55 percent), John Heinz of Pennsylvania (48 percent) and Bill Cohen of Maine (48 percent).

Past Senate Republican conference chairmen who would fail the purity test include John Chafee of Rhode Island (30 percent lifetime), Bob Packwood of Oregon (42 percent lifetime) and Margaret Chase Smith of Maine (33 percent in her last year). And that’s not counting the real liberal Republicans, an extinct species that included giants such as Jacob Javits (zero percent).

All but the most conservative Republicans in the Senate are on the run. George Voinovich of Ohio (69.8 percent), Judd Gregg of New Hampshire (78.7 percent) and Kit Bond of Missouri (81.9 percent) are all retiring. Arlen Specter (43.6) was forced from the party. Maine Sens. Olympia Snowe (47.9) and Susan Collins (49.4) are prime targets for future primary challenges.

That leaves, in addition to the still-unrated Scott Brown of Massachusetts, a couple of old bulls (Indiana’s Dick Lugar at 77.3 percent and Iowa’s Chuck Grassley at 83.5 percent) and a couple of Tennesseans who should be very nervous (Lamar Alexander at 79 percent and Bob Corker at 83.3 percent).

I suspect many conservative activists reading Milbank’s column thought, “Yep, they’re all just too liberal, and deserve to be purged.” But that only reinforces the larger point that’s been evident for a while: the party keeps moving further and further to the far right, and the hysterical party base just keeps demanding even more ideological purity.

And the more voters reward the party despite its ideological extremism, the less of an incentive the GOP will have to move back towards the middle.

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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.