In his recent article in Slate, historian David Greenberg shows a fundamental failure to understand the current situation in the Republican Party, taking the common viewpoint of many on the left that defectors like Arlen Specter have been driven from the Republican Party because of an intolerance of liberalism in the GOP. That interpretation may appeal to Democrats who want to portray Republicans as intolerant and dominated by the religious right, or who are still afflicted with Bush Derangement Syndrome, but it is a simplistic dismissal of Specter's own failures which led to his loss of support within the party and displays a profound ignorance of where the GOP is now and where it is going.
Greenberg does not understand the changes which are going on inside the Republican Party that made Specter increasingly incompatible, nor does he understand that hostility to Specter had very little to do with his rather flaccid liberalism and everything to do with his failure to adhere to any of the values which any of the still-diverse constituencies within the GOP believe in. What has turned other Republicans against Specter, and led to support for a primary challenge against him, is not his support for the arts or other "liberal" causes. It is his unwillingness to stand firm on fundamental Republican principles of smaller government and protection of individual liberty.
The GOP can be tolerant of someone who is somewhat socially liberal if they remain fiscally responsible. The problem with Specter is that while he was forgivably liberal on some social issues, he was unforgivably unreliable on issues of spending and responsible government. You can be a Republican and be a little bit socially liberal, and sadly you can be a Republican and get away with a bit of excess spending, but in the post-Bush environment you absolutely cannot remain a Republican and do both.
Critics on the left, like Greenberg, are misled by their outsider's view of the situation in the GOP. They keep on attacking the GOP as if it was the party of George W. Bush, but it hasn't been his party in a long time – if it ever was. They want to transfer blame for all of Bush's bad policies to the GOP as it now exists, choosing not to understand that most of the party only went along with him reluctantly in the first place and that the Bush’s failures and the Democrat win in 2008 have empowered elements within the party which have a very different agenda.
What Greenberg and others don't understand is the level of anger with Bush within the GOP and how widespread the desire to remake the party is. Bush's failures are why Obama is in the White House and Republicans are in the minority. Do you think Republicans like being out of power? Do you think they appreciate Bush for putting them there? Do you think those who survived this long don't realize their future survival depends on them being as un-Bush as humanly possible?
Specter sees this new environment and knows it is not going to be as tolerant of those who place politics above principle. His answer is to move to the Democratic Party, which is acting more like Bush and his politics of expediency every day. More principled Republicans realize it's time to institute reforms and move back to a more principled version of the party with a clear message that emphasizes broad policy differences with the Democrats on basics like taxes, spending, and personal liberty, rather than pandering to special interests, which went on under Bush, and which the Democrats have enshrined in their agenda.