One criticism of Arlen Specter I don't quite get, at least coming from liberals, is that his party switch reflects poorly on his character. Glenn Greenwald and Jon Chait, who don't actually agree on all that much, respectively call him "soul-less", and an "unprincipled hack".
Of course this is true, in so far as it goes. Specter has not been shifting to the left gradually over time, as one might expect from someone whose ideology was slowly "maturing". Rather, according to DW-NOMINATE data, he's actually moved slightly to the right in recent years, along with the rest of the Republican Party.
But if you're a Democrat, would you really want Arlen Specter to be anything other than a soulless, unprincipled hack? If Specter were more concerned about self-consistency -- and less about self-preservation -- he'd probably still be a Republican right now. Moreover, Democrats had better hope that Specter is as nakedly power-hungry as possible, because his best move from the standpoint of self-preservation is probably not merely to become a Democrat but to become a reasonably liberal one, along the lines of Bob Casey Jr.
There are, of course, two things you can do with a politician whose views you disagree with. The first thing is to elect him out of office; the second is to apply pressure -- whether moral, intellectual, political, or financial -- to get him to change those positions. In this case, the pressure placed upon Specter seems to have worked! True, we do not yet know exactly which positions he'll be changing along with his party label. He says he won't be changing his position on EFCA, for instance -- ironically, this is probably because he just flip-flopped that very issue last month. But the odds are very high that he'll be changing on at least some, reasonably important issues.
Voters, of course -- and human beings in general -- have a strong revulsion to inconsistency. When I'm trying to decide who to vote for, I factor in not only the articulated positions of the candidate but also an assessment of how likely he is to change them (in ways that I do not like). Past history weights heavily here. I never warmed to John Edwards, for example, because the campaign he was running in 2008 -- although I preferred his positions on issues like health care to the other candidates -- was fairly inconsistent with the one he had run in 2004, and far more inconsistent with the fairly conservative voting record he had accumulated in the Senate.
Then again, there is an odd kind of predictability about someone who is so craven as to constantly be shifting his position with the political winds. Just look up the latest Gallup polling and there you go!