The big caveat that should come with any discussion of 2016 is that a lot can happen between now and then. There may be one or two important 2016 candidates, whatever their ideologies, who remain obscure today. Or a prominent figure from Hollywood, Silicon Valley or a labor union could run.

Consider that eight years ago Mr. Obama could well have sat next to you on a plane without your noticing. At the time, he was an anonymous Illinois state legislator and Senate candidate. Herman Cain, who briefly dominated the current Republican race, was almost as obscure only a year ago.

Quick ascents are not wholly new. Abraham Lincoln and Jimmy Carter also went from national obscurity to the presidency in a short span. But such rises do seem more common than they once were. Twitter, Facebook and the rest of the Web allow candidates to jump ahead of others who have spent years wooing local party officials and editorial writers. The Supreme Court’s 2010 ruling relaxing campaign-finance regulations may help newcomers, too.

The current partisan rancor has also increased the benefits of not having a long paper trail, in electoral politics and beyond. Tom Daschle, the former senator, urged Mr. Obama to run in 2008, partly because he did not yet have a long voting record. Arguably the two most powerful unelected officials in the country are John G. Roberts Jr., the chief justice of the United States, and Ben S. Bernanke, the Federal Reserve chairman. A decade ago, Mr. Roberts was a lawyer in private practice, while Mr. Bernanke had run nothing larger than the Princeton economics department.

A plausible newcomer for 2016, Democrats say, is Elizabeth Warren, who advised Mr. Obama on the creation of a consumer-protection bureau for financial products and is now running to be a senator from Massachusetts (against Scott P. Brown, the incumbent and a Republican hero). She can deliver a punchy case for economic fairness, which has already made her a YouTube sensation. Obviously, she first would need to unseat Mr. Brown.

What kinds of Democratic candidates might we see in the 2016 field?

THE SILENT FRONT-RUNNER 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.