Douthat writes, and I agree, that if Obama doesn't magically fix the economy (through essentially the same strategy outlined by Bush but somehow seemingly idiotic then), or at least show significant improvement, then the situation may arise where a third party Presidential candidate (Ron Paul of course comes to mind) might be viable.
I see the situation going like this: the Republicans are totally hosed. Not only are they foundering and seem incapable of redeveloping leadership or even maintaining rational thoughts, but also their major demographic, the old white people, are dying off in growing numbers (or not voting). And any leaders the Republicans seem to bring forth are either old-school (read: untrustworthy) or apparently unable to outfox obama (read: incompetent).
Meanwhile, Gen Y (people born between 1980-2000) is growing vote-eligible and fast. By 2012, slightly more than 60% of Gen Y will be old enough to vote, and by 2016 over 80% of Gen Y will be eligible. That's a huge influx of young voters, on the order of 15-20 million. Especially compared to Bush 43's first term, where only 20% of Gen Y could vote.
Depending on whom you ask, the Gen Y'ers voted either favorably or overwhelmingly for Obama, because they were highly critical of Bush and the Republican Party, either due to his war (where Gen Y was exclusively fighting) or his mishandling of the economy, or his gross misuse of the Christian faith as a justification for unethical behavior (or his unethical behavior in spite of his professed Christian faith). So Obama is probably hoping (expecting) to carry Gen Y again in 2012. But I often argue that Gen Y is the "instant gratification generation"; if President Obama can't carry through all his claims and quickly, there's a decent chance that a savvy candidate could turn Gen Y against him.
And as Douthat notes, Perot pulled his percentage of the popular vote by making the Republicans look untrustworthy and the Democrats look like failures. I could easily see Ron Paul do the same, especially if the economy isn't in significantly better shape. Ron Paul has a library of sound bytes predicting the market crash, proposing solutions, railing on Democratic leadership and Republican leadership, and that could easily be used as an attack on the failed strategies used (by both parties) to rebuild the economy.
Now, I am not currently a Ron Paul supporter. Most of Ron Paul's solutions to problems seem way too aggressive and dramatic to be feasible. Right or wrong, he mostly represents "the obvious solution" that cannot be achieved. Like the obvious solution to global warming is to annihilate 90% of the human species, but that's not exactly practical. A quick browse of this blog shows that I voted for Obama, partially to stop John McCrank from getting elected, but also because I, like many of my peers, deeply bought in to Barack Obama and his seemingly genuine devotion to improving the country. So perhaps I (along with any others who were Gen Y Obama voters but not worshippers) are in a position to quickly turn on the President if his promises are not kept, or if his goals are not met. Perception of betrayal can be a dangerous emotion.
And should that happen, that the old folks still vote for their beloved Republican party, and the young folks abandon the innocuous Democrats and Obama, expect Gen Y to turn not to the Republicans, but to a potential third party candidate who aggressively and proactively responds to a new generation and their requirements for a public candidate.
However, I don't seriously think that Ron Paul, or any other third party candidate, could be a viable candidate to actually get elected in 2012. I think they easily receive a sizeable majority on the order, if not greater, than Ross Perot achieved in '92. But I don't see a third party candidate going over 15% yet. And I refuse to make 2016 predictions.
And if a third party candidate in fact got elected in 2012, it could be a total disaster; instead of opposition on one side of the aisle, he or she instead faced the wrath of both sides.