Ever since Obama took office, the anti-war left has shrunk to just Progressives.
It's natural in a two-party system for a party who has been fighting the other side being in power, having some exhaustion when 'their side' wins and is too 'centrist'.
Partly it's just a lack of an alternative, and the risk that attacking 'your guy' can have the unintended consequence of helping the far worse 'their guy' get elected.
In other words, compromise on Obama, to prevent the far worse next George Bush.
However, that has the effect of almost guaranteeing 'your guy' will do the political effective thing of 'governing from the middle' because whether he has your vote as you are crazy about him, or he has your vote as 'can barely stand him but he's better than the other side' is the same one vote, but by moving to the other side he might actually pick up some votes.
The left widely felt that FDR was not liberal enough; he told them, make him do what they want, meaning, change the public opinion to where it demanded he do the liberal policy.
Obama has said the same thing.
And the right has some valid criticism - made by the left as well - when it points out the double standard that Obama on some policies is 'as bad or worse than Bush'.
The point was recently made with the Patreaus appointment and Moveon's apparently very different reaction under Bush and Obama.
But the right usually fails to note the overall policies, and just leaps to note this behavior as if the issue of the left criticizing Obama could help elect the next Bush isn't important.
In other words, criticize the specific things, like giving Obama a pass on something, but recognize it can be just fine to vote for him anyway compared to the other guy.
The liberals have a lot of valid criticisms of Obama that some of his policies are to the right of the general public - for example, who would say Obama has gone after Wall Street as much as the average American would like - but it's also fair to ask, would the Republicans be the Wall Street attackers if they were in power?
As Rachel Maddow explained last night, the game is for the party out of power to simply attack the party in power, and expect the public to vote for them as if they're better, even though they're perhaps worse. I'm sure many Republicans would agree with this point, when it comes to suggesting some who were fed up with Republicans voted for a Democratic congress they think is worse (and which I think is much better).