Nate Silver explains:

The way that the Republicans achieved that big swing in 1988, assisted by a couple of significant gaffes from the Dukakis campaign, was to portray Dukakis as too liberal for the American mainstream. The same basic strategic template was employed against John Kerry in 2004. However, this strategy is unlikely to work in 2008. How come? Barack Obama is already perceived as being very liberal.

And Bush and Rove have made liberal cool again.

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