But the recent history of partisan politics should serve as a reminder of how quickly fortunes can turn. After the '64 election, the Republicans were thought dead. Post Watergate '73-'74, Nixon had so poisoned the Republican brand that a number of members of congress changed parties as the only hope of retaining their seats. Even as recently as the '92-'93 early Clinton popularity the Republicans were considered irrelevant. And the Democratic party has been written off for dead more than a few times. McGovern and the hippies were thought lethal, St. Ronnie led to the migration of the "Reagan Democrats", and a consensual blow job was considered such a moral outrage that no upstanding citizen would ever consider voting Democratic. Karl Rove thought that 9/11 could be used to create a "permanent Republican majority" and render the Democrats "irrelevant".
My point is that political fortunes turn, sometimes very quickly. The bad ideas and conservative constituencies that are the base of the Republican party still exist. The corporatist oligarchs still have the resources to promote their self-enriching deregulation agenda to the middle class, despite the fact it's been shown to be a total failure. The religious right can still move a large number of voters by demonizing gays and abortion. Characterizing Democrats as "tax and spend liberals" will still resonate with a percentage of voters, and there is always a market for the NRA's "liberals want to take away your guns" lie. So the people that bought Shrub could get fooled again.
Another factor that must be taken into account is that Obama was elected partly because of his charisma. While none of the current faces leading the Republicans can rival that, a charismatic individual could emerge in the future and rally enough voters to win. It's not like that's never happened before.