The state of the economy, undoubtedly, will be a huge part of the equation. The vast majority of economists expect it to continue to grow through 2012; the forecasts, in fact, have become slightly more optimistic over the past several weeks. The bad news for Mr. Obama is that the forecasts are more optimistic about G.D.P. growth than they are about unemployment: economists also expect the employment picture to improve, but at a sluggish pace, with the unemployment rate most likely being in the low 8 percentage point range at the time voters go to the polls in 2012. How voters might react to this situation — one in which the employment picture has improved, but is still rather poor in an absolute sense — is an open question, and one for which the historical evidence is of relatively little use.

How about the Democrats’ performance during the lame-duck session of Congress, when they were able to pass several important bills like the repeal of the military’s “don’t ask don’t tell” policy, to the surprise of many observers?

One needs to be careful not to unintentionally damn Mr. Obama and the Democrats with faint praise; yes, they got a lot done, but all of the measures they passed were quite popular. Once the Republicans take over in the House, Mr. Obama will be engaged in some big showdowns with them over issues like the budget and his health care plan. Mr. Obama will be fighting from a defensive posture on health care, which remains unpopular with the public. How the public feels about the budget, where it has little faith in either party, is less clear.