Political analysts and Beltway pundits can debate the obstacles facing Democrats in the 2016 presidential election. But there's one inescapable truth facing the party's nominee, no matter if it's Hillary Clinton or Elizabeth Warren or Julian Castro: Voters rarely elect someone from the same party for consecutive terms.

“Mostly, the White House flips back and forth like a metronome. Voters just get tired after eight years,” the writer Megan McArdle wrote. Explains political analyst Charlie Cook: "They tend to conclude that it is 'time for a change,' and they trade the in party for the out party."

In fact, since American politics evolved into what we know as the current two-party system, the last time voters elected a Democrat to the White House after a president from the same party had just served a full term was in 1856, before the Civil War. If that's not enough to scare of presidential hopefuls in the Democratic Party who want to succeed two-term President Barack Obama, what is?

Last Democrat to Succeed a Democrat

The last Democrat elected to succeed a Democratic president was James Buchanan, the 15th president and the only one ever to come from Pennsylvania. Buchanan succeeded President Franklin Pierce.

You'd have to go back even further in history to find the most recent instance of a Democrat being elected to succeed a two-term president from the same party. The last time that happened was in 1836 when voters elected Martin Van Buren to follow Andrew Jackson.

This, of course, does not include the four terms of Democrat Franklin Delano Roosevelt; he was elected to the White House in 1932 and re-elected in 1936, 1940 and 1944. Roosevelt died less than a year into his fourth term, but he is the only president to have served more than two terms.

Why It's so Rare

There are very good explanations for why voters rarely choose a president from the same party for three consecutive terms. The first and most obvious one is fatigue with and unpopularity of the president who is completing his second and final term at the time of the election for his successor.

That unpopularity often sticks to the candidate of the same party. Just ask some of the Democrats who sought unsuccessfully to succeeded Democratic presidents including Adlai Stevenson in 1952) Hubert Humphrey in 1968 and, most recently, Al Gore in 2000.

Another reason is distrust of people and parties who hold power for too long. "The distrust of people in power ... dates back to the age of the American Revolution and the mistrust of hereditary rulers with no curbs on their powers," wrote the National Constitution Center.

What It Meant in 2016

The rarity of presidents from the same party being elected consecutively is not lost on political analysts when it came to the 2016 presidential election. Many believed the success of Hillary Clinton, who was the most likely contender for Democratic nominee, hinged on who the Republicans chose.

Opined the New Republic:

"The Democrats could benefit if the Republicans nominate a relatively inexperienced right-winger or someone who possesses the temperament of a high school football coach rather than a president ... If they opt for an experienced centrist in 2016 — Florida’s Jeb Bush is the obvious example — and if the party’s right wing doesn’t demand he toe the line, they could stand a good chance of reclaiming the White House and of confirming Americans’ reluctance to keeping the same party in the White House three terms in a row."