It's been puzzling so far, but I think I figured out the Republican strategy for the presidential election. Obama is already giving them everything they want in terms of policy, pretty much, and there's no reason to think he'll change after the election. Centrism is not an act for Obama, it's who he really is and when you compromise from the middle, and do so poorly -- i.e. give up most demands for one or two centrist bits in the final legislation -- the outcome leans heavily to the right. Given that, it's not at all clear that Republicans would be able to reach more of their goals if they controlled the presidency and faced united opposition from the left rather than the fractured opposition they face under a Democratic president.
And the right certainly has nothing to complain about when it comes to war mongering, domestic spying, and the like. Republicans are getting everything they want, more than they ever could have dreamed of with a Democrat in power, with little outcry from the leftists that usually make such a fuss over these types of activities.
Plus, the president is taking the blame for the lousy economy, and they are able to make headway in their long held goal of discrediting Keynesian economics. Having a president who only half-heartedly at best supports Keynesian policies -- witness the pivot to deficit reduction at the drop of a hat when it looked politically convenient -- means that the outcome of the policies will be as lackluster as Obama's commitment to them. So long as Republicans can block more effective policy, and so long as the president acquiesces and doesn't blame Republicans for obstructionism, etc., Republicans can win this war. That socialist, Keynesian (or is it Kenyan?) president tried it his way, and it failed!
If a Republican takes the presidency, that blame will fall on them. Sure, it's possible that the economy will take off over the next four years, but more realistically it's looking like more of the same sluggishness, a slow, agonizing, "are we there yet," recovery that won't be helped at all by the deficit reduction that lies ahead (thanks to one of those wonderful compromises by the administration). Who wants to preside over that? Instead, why not put up some weak candidates, very, very weak, and give the win to Obama? He's already done a lot of their work for them, why not let him take the lead in dismantling social welfare in the name of deficit reduction (e.g. Obama has often cited Social Security as a target).
I realize that Republicans really do want to win, they hate having Obama in power, but from their perspective would it really be so bad if they lost?