Since we're about to leap into an unknown future tomorrow, I found myself wondering what Donald Trump's chances of being a one-term president would be. Historically, we're already in one of the longest runs of two-termers in all of American history, so if that's any indication, Trump's chances for two terms seem pretty remote.

Of course, this is all just idle speculation. Historical firsts are set all the time, so there'd be nothing stopping Trump from setting his own. He's certainly already defied the odds a number of times and come out on top.

When Barack Obama ran a second time, a good historical case was made for why he wasn't going to win a second term. Few presidents have ever been re-elected with the unemployment rate so high, for instance (it was still stubbornly above seven percent during the campaign). Obama's job approval rating stayed below 50 percent for most of his re-election campaign as well, which normally means a loss at the ballot box. And America had only ever seen three two-termers in a row once previously -- and that was during the absolute collapse of the first real political party in the country (the Federalists, who were decimated by their opposition to the War of 1812). So the odds were stacked against Obama, but he won anyway.

This means we've now had two periods of three two-termers in a row. The first was Thomas Jefferson followed by James Madison and James Monroe. This was also notable because all three were from the same party -- a 24-year run that became known as the "Era of Good Feelings" (even though it really wasn't). Monroe actually ran unopposed for his second term, something that will likely never happen again. Bill Clinton, George W. Bush, and Barack Obama were the second run of two-termers, but (obviously) there were two party switches in there, from Democratic to Republican and then back again.

If you count a different way, America has seen an even longer run in the twentieth century. If you count presidents who died in office and were succeeded by their vice presidents as the same term, then we had an extraordinary run from the Depression to the 1970s. Franklin Roosevelt won four straight terms, but most of his fourth was served by Harry Truman after Roosevelt's death. By this logic, Roosevelt/Truman served five actual terms, after Truman won his own election outright. This duo was followed by Dwight D. Eisenhower, who served a full two terms. John F. Kennedy didn't finish his first term, but then Lyndon Johnson won after completing J.F.K.'s first term, so Kennedy/Johnson served two terms altogether. Then Richard Nixon won two terms, the second of which was finished by Gerald Ford after Nixon resigned. Taken together, this means we had a five-term team, a two-term president, a two-term team, and a final two-term team. That's the longest run without either party having a single term in office in all of American history, but the whole "counting the vice president" idea is kind of bending the rules a bit, I realize.

If Trump does win a second term, it'll be the longest run of two-term presidents ever. This doesn't mean it won't happen, but the historical odds are certainly against it. External events have a way of messing up neat historical parallels, and nobody has any idea what will happen during Trump's first term. When George W. Bush was sworn in, nobody expected him to be a war president, to give the most obvious recent example. Who knows what situations Trump will face, and who knows how he'll react to them? His public opinion could go through the roof (as George W. Bush's did, after 9/11) or could plummet, as a result. Even a spike upwards in public opinion is no guarantee of re-election, though, as George H. W. Bush learned (his approval rating also went through the roof during the Gulf War, but then crashed back down when Republicans punished him for abandoning his "no new taxes" pledge).

Most presidents these days have a built-in "floor" for their job approval numbers. Roughly 40 percent of the country will approve of a president of their own party, no matter what he does or doesn't do. That's normal. Barack Obama's job approval rating stayed above 40 percent for his entire two terms. Presidential job approval ratings are usually a measure of how the independent 20 percent of the population views him, in other words.

Trump's built-in floor seems a little lower, however. Personally, I'd put it at around 35 percent, if not slightly less. I think there are a whole lot of Republicans who are happy enough that a Republican will be in the White House right now, but who could walk away from Trump if he disappoints them at some point in the next year or two. This happened to George W. Bush when the Iraq War went sour and dragged on for much longer than anyone anticipated, and then Bush's approval ratings truly tanked when the economy fell apart in his last year in office.

Trump's core supporters will doubtlessly continue supporting him (and believing his interpretation of things) unless he flip-flops on some major promise he made to them in some way (as happened with the "no new taxes" problems the elder Bush had). If Trump does something unexpected that earns him the ire of the conservative Republican establishment, he'll likely see his approval dip below 40 percent. Maybe Trump makes a deal with congressional Democrats, or maybe he insists on supporting something that conservatives hate -- it could happen a number of ways.

The other way Trump could lose support is if an external event happens that he can't effectively deal with. The business cycle could be overdue for a downturn, and economic bad news is one of the prime reasons many previous presidents only saw one term in office. Or if Trump is faced with a foreign policy crisis and botches it badly, public support could disappear very quickly, even among Trump's core supporters. If his actions veer away from his rhetoric in a big way, many will stop giving him the benefit of the doubt.

Of course, the opposite is just as possible. Trump could make some bold and decisive moves and begin picking up the support of the independents in a big way. Again, this could happen a number of ways -- perhaps he'll introduce some creative plan to help small American towns hit hard by the loss of manufacturing jobs, or force some foreign country to bend to his will in some way (military or economic, perhaps). If Trump manages to score some big successes that are wildly popular, then many people who now can't support him may come around to doing so.

Trump will likely need something on these lines if he does manage to get re-elected. The promises he made campaigning the first time were so vast and sweeping that he really will need to make good on at least a few of them if he's going to have any chance of a second term.

I personally have no idea what the odds are that Trump will be seen as a success by the public or not, at least to the level of winning re-election. He enters office with a very low approval rating, but it is roughly at the lowest point that Obama ever saw. Obama came back from low polling a number of times, although his job approval rating didn't climb over 50 percent until just before the 2012 election happened.

So the odds right now seem to suggest that Trump will have a very hard time winning a second term. History also suggests this, since we've gone a very long time without having a one-term president. Trump himself might even get tired of the job and decide one term is enough. That would be pretty surprising, since it so rarely happens. But, as I began with, this is really all just idle speculation, since Trump has proven one thing already -- he has defied expectations in a big way, and he might just continue to do so in four years' time, one way or another.

-- Chris Weigant

Follow Chris on Twitter: @ChrisWeigant