The Radical Centrist asserts, and Ambivablog/The Yellow Line concurs, that centrists take more flack from the left than the right. Is this true? I'm not so sure.
I can only operate off personal narrative here. I'm a centrist-liberal, who resides in a very liberal locale (Bethesda, MD), and attends a very liberal school (Carleton College). The particular centrist positions I take are that I support the war in Iraq, straddle the fence on abortion, and generally believe in an aggressive and hawkish foreign policy (albeit one that stressed democracy). So basically, its not like I'm lacking in ways to piss hardcore liberals off. Yet I've never really felt uncomfortable amongst my liberal peers for holding these positions. Sometimes after mentioning them, I have to inform them that I actually am a Democrat, just a moderate one. But I've yet to meet the reflexive hostility that seems to be what people are talking about when they refer to what the stalwarts supposedly feel about the moderates (for example, I've never been called a traitor).
I think what is obviously true is that the left is more tolerant of rightwingers turned centrist (for example, Andrew Sullivan and John Cole) than they are of liberals who appear to be drifting off to the center (for example, the DLC). For example, I've seen The Daily Kos link approvingly to John Cole on several occasions, while bashing the DLC on, well, more than several occasions (despite the fact that the DLC is probably more liberal than Mr. Cole). The reverse is also true--Republican partisans are far more kind supposed moderate Democrats than they are to moderate Republicans. This makes perfect sense, after all: we like people who seem to be moving in our direction, and are upset with those who appear to be moving away from us. But I don't see any partisan slant to the phenomena.
As a general matter, I'd think that moderates are more welcome in the Democratic party than in the Republican party, and I think that bipartisanship is slightly more common their as well (though in today's environment, it's rare anywhere). For example, while I've seen bona fide liberals praising Bush (for specific policies, of course) on several occasions, I have yet to see any comparable praise from a mainline Republican commentator of a mainline Democrat. The moderate wing of the Democratic party is far more powerful than it's equivalent within the Republicans (DLC stomps Rockefeller Republicans). How else do you explain pro-life Harry Reid (and his predecessor for that matter, Tom Daschle)? Neither of them are all that liberal, objectively. Nancy Pelosi certainly is, but Steny Hoyer isn't. What you have for Democrats seems to be a fair mix between the left and center of the party. The big Republicans in congress, by contrast, are Tom DeLay, Roy Blunt, Rick Santorum, Jon Kyl, and Bill Frist--all toward the right edge of the party, and only Frist even sporadically nodding to the center. Moderate Republicans never even see the light of leadership day--remember how Arlen Specter was humiliated before finally getting the chairmanship of the Judiciary committee (which is just because of seniority, anyway)? The other moderates, like Collins, Snowe, and Chaffee are completely out of the loop (though again, only Voinovich has been labeled a traitor) Finally, I think it is telling that more moderate Republicans appear to be bolting the party (even if only temporarily) than moderate Democrats doing the same. Sullivan, Daniel Drezner, libertarian Jacob Levy, among others, all voted for John Kerry in 2004. Senator Lincoln Chafee refused to say he was voting for Bush, and of course, Jim Jeffords defected precisely because he was fed up with being abused by his own party. With the exception of Zell Miller I have seen almost no similar crossovers from the Democratic party.
So while I think that there is a lot of illwill at the extremes to the center on both sides, I think it is descriptively inaccurate to say that it resides on the left. At best, it is an equal opportunity offender, and at worst it is a problem more acute on the right.