Republicans did well in the midterm elections, but there is widespread agreement that they face a demographic disadvantage in the 2016 presidential election, when many of the predominately Democratic younger and minority voters, who stayed home in 2014, will return to the polls. It’s true that an expected increase in turnout will benefit the Democrats, but may not be enough to elect another Democratic president.

The chief obstacle that any Democratic nominee will face is public resistance to installing a president from the same party in the White House for three terms in a row. If you look at the presidents since World War II, when the same party occupied the White House for two terms in a row, that party’s candidate lost in the next election six out of seven times.

The one exception was George H.W. Bush's 1988 victory after two terms of Ronald Reagan, but Bush, who was seventeen points behind Democratic nominee Michael Dukakis at the Republican convention, was only able to win because his campaign manager Lee Atwater ran a brilliant campaign against an extraordinarily weak opponent. (Democrats might also insist that Al Gore really won in 2000, but even if he had, he would have done so very narrowly with unemployment at 4.0 percent.)

There are three reasons why the three-term obstacle has prevailed. The first and most obvious has been because the incumbent has become unpopular during his second term, and his unpopularity has carried over to the nominee. That was certainly the case with Harry Truman and Adlai Stevenson in 1952, Lyndon Johnson and Hubert Humphrey in 1968, Gerald Ford (who had succeeded Richard Nixon) in 1976, and George W. Bush and John McCain in 2008.

The second reason has to do with an accumulation over eight years of small or medium-sized grievances that, while not affecting the incumbent’s overall popularity, still weighed down the candidate who hoped to succeed him. Dwight Eisenhower remained highly popular in 1960, but some voters worried about repeated recessions during his presidency, or about his support for school integration; Bill Clinton remained popular, and unemployment low, in 2000, but his second term had been marred by the Monica Lewinsky scandal, and coal-state voters worried about Democrats’ support for Kyoto while white Southern voters worried about the administration’s support for African American causes.