Colbert and Stewart create hour of parody every night that mocks the GOP and Conservatism. It's been underrated as one of the reasons why the electorate, especially the young, have gone left in recent years. It's tough to survive in politics when you're a laughingstock, and that's what the GOP has become, thanks to their two tv shows.
But the main reason is, of course, the failure of the Dubya administration. He was one of the worst Presidents ever, and by the end of his eight years, "The Tough on Terror Party of Fiscal Responsibility" was losing two wars and giving welfare to Wall Street.
Conservatives only have themselves to blame, and it's clear on this message board that they haven't gotten around to doing that yet.