The recent liberal candidates' failures are to be attributed less to their liberalism than their poor campaigns. Think back to 2000 and ou'll notice that Gore's and Nader's combined vote outdoes Bush's and Buchanan's by 2,977,696--somewhat indicating a desire for a liberal president. In 2004 Bush was re-elected with the narrowest PV percantage margin in history and 59,028,548 votes amounting to 48.27% of the PV isn't really a crushing defeat either.
For the time being, Bush may have succeeded at taking the country to the right, but that does not mean that a liberal has no chance. If the Dems only nominated a strong candidate with substance and learned to run a proper campaign there is a good chance he could pull it off. (And Feingold is a candidate who could do just that, so don't isolate him in the VP slot.)
I disagree. The country has been swinging to the right for a long time- since 1968, in fact. This isn't personal support of Bush; he actually did a lot worse than he might have.