President Obama's greatest setback to date has been the 2010 midterm elections. Gains that Republicans scored in the House and Senate still circumscribe his agenda. It is no surprise, then, that the Obama White House wants to achieve something no other president has ever done: Retake full control of Congress in a midterm.

The party of an incumbent president traditionally loses seats in midterm elections. The usual strategy is simply to minimize the damage. Yet Mr. Obama and many Democrats are so buoyed by national polls and...