If Americans send President Obama back to the White House for a second term, it won’t be pretty.

And in fact, it will be almost without precedent.

A hard-fought Obama win — which is about the only way he’s going to win — would likely make Obama just the second president to be reelected by a smaller margin than he won in his first race.

The history of American politics demonstrates that presidents who seek a second term either lose outright or win reelection with an even bigger mandate than they had before.

Below are the 11 presidents who have won reelection, comparing their first margin of victory to their second margin. Another eight presidents lost reelection.

Polls (and the economy) today demonstrate that Obama will be hard-pressed to match or exceed his 7 percent popular vote margin of four years ago; it’s just not going to happen barring something major. So if he wins, he’s likely to join Woodrow Wilson as the only presidents to win by a smaller margin the second time around.

(Update: A Fix reader notes that Wilson is something of a special case. He won by a large margin his first time in large part because Theodore Roosevelt split the GOP vote by running as a third-party candidate. Wilson actually increased his share of the vote between 1912 and 1916 by seven points.)

What does that say about the American electorate?

It’s hard to say with any certitude. The fact is that, even though 19 presidents have sought reelection and 11 have won, that’s still a pretty small data set to draw any broad conclusions.

But the data suggest that voters have rarely reelected a president who hasn’t lived up to their initial promise. If a president is losing voters from his first race, he is almost always losing reelection too.

But that may be also be due to the advantages of incumbency. Many presidents win their first term by small margins because they aren’t as well-known. If they succeed and people get to know them, of course more people are going to vote for them the second time.

Obama had a relatively big win his first time around (at least as far as today’s polarized electorate is concerned). Matching that win was a tall order from the beginning.

But the fact is that most presidents in Obama’s position — that is, in worse shape than in their first race — go on to lose.