It is largely due to this hypocrisy that the American people were so hungry for change in 2008. Even many who would normally not agree with Mr. Obama's policy positions felt that something different was needed in Washington; any change had to be better than more of the same. Thus the American people elected a man with unwavering faith in the superior capability of Big Government, a man who believes in the redistribution of wealth and supports abortion on demand, a man who is a reliable friend to organized labor and the environmental lobby, and a proponent of nationalized health care. In short, the American people elected a Democrat.
If Obama had won the Presidency under this mantle only to slash entitlement spending, appoint an anti-Roe justice to the Supreme Court, ignore his Speaker's cry for comprehensive health care reform, and backpedal on the push for cap-and-trade legislation, he would have been painted as a hypocrite and a disgrace to his party. There are assumptions that people make about what it means to be a Democrat, and the agenda that a Democrat is likely to pursue while in office. The same is true of Republicans. This is why political parties go to the trouble of drafting a party platform in the first place, to clarify what they stand for, what principles guide their leadership and inform their decisions, and how they view the relationship between citizens and their government.
How one answers these questions determines (in America, anyway) on which side of the aisle one falls politically. But for far too long there has been little to no correlation between what GOP said it stands for and what it actually does. Thus this "ideological cleansing" of which Milbank complains may actually help eliminate the cognitive dissonance that the American people have experienced as a result of Republicans saying one thing and doing another.
If the GOP is successful in achieving basic ideological unity within its party, then the American people will have an easier time determining if the conservative approach to government is something they support, and the charlatans inside the Beltway will have a harder time gumming up the works with politics as usual.