First, Mr. Obama’s personal favorability ratings — which continue to average about 50 percent — are considerably higher than his approval ratings, which are now around 40 percent. It’s not uncommon for favorability ratings to track a point or two ahead of approval ratings — but this is a particularly large gap. Voters remain reasonably sympathetic to Barack Obama, the person, even if they’re growing less and less thrilled with his performance…

Nor is it likely that another Democrat would advocate policy positions that differed substantially from Mr. Obama’s, which are very close to those of the typical Democrat in Congress. Few Democrats argued against the need for economic stimulus. The health care proposals advanced by Ms. Clinton and John Edwards during the 2008 campaign were not much different from Mr. Obama’s now-unpopular one. Few Democrats have any ideas about how to substantially reduce unemployment that could pass through the Republican Congress.

So the message would be mostly the same — but would be delivered by a Democrat who was probably no more effective a messenger than Mr. Obama, and who would lack the aesthetic and tactical advantages of being an incumbent president.