Although it’s true that Democrats have fared well in recent presidential elections, there is no guarantee their success will continue. In 2016, the Democratic nominee will have to argue for a third term for the party at a time when Mr. Obama’s approval ratings may be beneath 50 percent, and when many Americans may be anxious or downright pessimistic about the country’s future. At the moment, Mr. Obama’s approval ratings and the pace of economic growth are consistent with a close, competitive race, in which the Republicans might have a slight but uncertain edge.

There were signs in last year’s midterm elections that dissatisfaction with Mr. Obama’s performance was costing Democrats among white voters without a college degree, most obviously in the countryside of Iowa and Colorado. Those 2014 results suggest that Republicans have a route to winning the White House, even if it’s a narrower one than the Democrats’ path.

Yes, Mrs. Clinton has a large lead over her likeliest Republican challengers in national polls, including in many of the most important battleground states. But general-election polls more than a year before Election Day have little meaning. More specifically, the preponderance of undecided voters in those polls have been Republican-leaners who will eventually rally behind the Republican nominee.

Recent Fox News, ABC/Washington Post and Marist polls found Mrs. Clinton with a large lead over Jeb Bush, for instance, but Mr. Bush earned no more than 81 percent of the support of Republicans (and 69 percent among conservatives) in any of the three surveys. Mrs. Clinton held at least 87 percent support among Democrats. The conservatives who are not backing Mr. Bush in these very early polls are likely to come home to the party should he win the nomination.

There is also evidence, especially in state polling, that Mrs. Clinton’s support from traditionally Democratic, white, conservative voters is unsustainable — especially in the South and Appalachia. For instance, polls in Kentucky matching her against the home-state favorite Rand Paul have often shown her running well ahead of Mr. Obama’s performance in the state in 2012, and still further ahead of candidates from elsewhere.

On paper, her strength among these voters is her greatest advantage over Mr. Obama. Perhaps 10 or 15 years ago, Mrs. Clinton could have run a campaign to win these white, Southern conservatives and perhaps carry Arkansas or West Virginia, two states her husband won in both of his presidential elections. But that strategy is untenable in today’s Democratic Party, which will demand a far more liberal candidate than conservative voters are willing to tolerate.