Imagine for a moment how Democrats would feel late on the night of Nov. 6 if news outlets began to call the race for Mr. Romney or another Republican. The comedown from four years earlier would be one of the starkest in American political history. The promise of Mr. Obama’s victory would yield to the reality that a Republican president, and probably a Republican Congress, would be likely to undo significant parts of his agenda, starting with aspects of health care reform.

The post-mortem is easy enough to predict: Mr. Obama was a lamb among lions, naïve to believe that he could win Congressional Republicans over to major bills, even compromises. Mitch McConnell, the Republican leader in the Senate, said as much early in Mr. Obama’s term…

If Mr. Obama loses, attention will shift to Mrs. Clinton almost immediately. She brings her own baggage, having run a troubled presidential campaign and been a leading figure in a somewhat chaotic White House. She also sounds sincere when she talks of wanting a break. Arguably, no public figure has had a more intense past 20 years.

But at 64, she remains energetic and politically attuned. With friends and close colleagues, she still talks passionately about how she believes the Republican Party is harming the country. Polls show that she is among the most admired people in the United States. Given all that, turning down the prospect of beating a Republican incumbent might not be so easy.