I saw Lincoln over the weekend and thought it was both great fun to watch and weirdly amateurish, as if Hollywood's A-list all decided to pitch in on a community-theater production about Abraham Lincoln.At the same time, I've been reading Stephen B. Oates' Lincoln biography, With Malice Toward None. Early on, Oates recounts Lincoln's courtship of Mary Ann Todd. Lincoln was, by any stretch of the modern imagination, a catch: Born to nothing and still a young man, he'd built a thriving legal practice, been elected and reelected to the Illinois legislature, and achieved some national renown as a Whig orator. In 1840, Lincoln and Todd "reached an 'understanding' and evidently became engaged." Then, her family stepped in and killed the romance. Lincoln, they thought, came from "nowhere," while the Todd family was wealthy and educated. Lincoln was told that he was no longer welcome to visit Mary, and the engagement, at least for a time, was off, throwing Lincoln into a particularly black depression.Lincoln and Todd would eventually marry. But what's remarkable about this vignette is the tensions you see in the American class system. On the one hand, there was -- and, to a lesser degree, is -- an American class system. The idea that the United States is a classless society is, and has always been, a myth. On the other hand, a guy who wasn't initially thought good enough to marry into the upper crust was nevertheless able to become president. That part of the American story is real, too.One accelerant to American social mobility has always been that our class system is weaker than the European systems, in part because our country is younger than the aged civilizations of Europe. But the European class systems have weakened. That means our accelerant is gone, and our historical edge in social mobility is eroding, too.The Pew Charitable Trusts' project on economic mobility looked at 16 studies and concluded that "in the United States, there is a stronger link between parental education and children's economic, educational and socio-emotional outcomes than in any other country investigated."Part of the genius of the American system has been the recognition that the country benefits from a less-ossified class of elites. Where the class systems of other cultures held that there was a certain segment of the population that was born to rule and that everyone would suffer if the common man rudely shouldered his way to the front, Americans have believed that we are better off if men like Lincoln -- born to illiterate parents in a log cabin -- had the ability to lead the nation. And we are right. Those who are born to rule often beget children who are born to spend their parents' fortune and besmirch the family name.The sweep of U.S. inequality is not as broad as it once was. But there is still a vast distance from the top to the bottom, and it's becoming harder to traverse here than it is elsewhere. In part this is because of what Christopher Hayes, in his book Twilight of the Elites, calls "the iron law of meritocracy," which holds that "eventually the inequality produced by a meritocratic system will grow large enough to subvert the mechanisms of mobility. ... Those who are able to climb up the ladder will find ways to pull it up after them, or to selectively lower it down to allow their friends, allies and kin to scramble up."America is a country that not only permits log cabin presidents but celebrates their humble origins. It's also a country that just reelected an African American president who was born to an absentee Kenyan father rather than put in office the son of a former governor. But it is also a country in which many are eagerly looking toward a 2016 presidential race in which the Republican front-runner is a Bush and the Democratic front-runner is a Clinton.Class is less binding today than it was in Lincoln's time, but circumstance is more binding than it was fourscore and seven years ago.Ezra Klein is a columnist for The Washington Post.