When we sat down in May, he explained how he comes to a decision. Lichtman's prediction isn't based on horse-race polls, shifting demographics or his own political opinions. Rather, he uses a system of true/false statements he calls the "Keys to the White House" to determine his predicted winner.

And this year, he says, Donald Trump is the favorite to win.

The keys, which are explained in depth in Lichtman’s book “Predicting the Next President: The Keys to the White House 2016” are:

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Party Mandate: After the midterm elections, the incumbent party holds more seats in the U.S. House of Representatives than after the previous midterm elections. Contest: There is no serious contest for the incumbent party nomination. Incumbency: The incumbent party candidate is the sitting president. Third party: There is no significant third party or independent campaign. Short-term economy: The economy is not in recession during the election campaign. Long-term economy: Real per capita economic growth during the term equals or exceeds mean growth during the previous two terms. Policy change: The incumbent administration effects major changes in national policy. Social unrest: There is no sustained social unrest during the term. Scandal: The incumbent administration is untainted by major scandal. Foreign/military failure: The incumbent administration suffers no major failure in foreign or military affairs. Foreign/military success: The incumbent administration achieves a major success in foreign or military affairs. Incumbent charisma: The incumbent party candidate is charismatic or a national hero. Challenger charisma: The challenging party candidate is not charismatic or a national hero.

Lichtman, a distinguished professor of history at American University, sat down with The Fix this week to reveal who he thinks will win in November and why 2016 was the most difficult election to predict yet. Our conversation has been lightly edited for length and clarity.

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THE FIX: Can you tell me about the keys, and how you use them to evaluate the election from the point where — I assume it's very murky a year or two out, and they start to crystallize over the course of the election.

LICHTMAN: "The Keys to the White House" is a historically based prediction system. I derived the system by looking at every American presidential election from 1860 to 1980, and have since used the system to correctly predict the outcomes of all eight American presidential elections from 1984 to 2012.

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The keys are 13 true/false questions, where an answer of "true" always favors the reelection of the party holding the White House, in this case the Democrats. And the keys are phrased to reflect the basic theory that elections are primarily judgments on the performance of the party holding the White House. And if six or more of the 13 keys are false — that is, they go against the party in power — they lose. If fewer than six are false, the party in power gets four more years.

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So people who hear just the surface-level argument there might say, well, President Obama has a 58 percent approval rating, doesn't that mean the Democrats are a shoo-in? Why is that wrong?

It absolutely does not mean the Democrats are a shoo-in. First of all, one of my keys is whether or not the sitting president is running for reelection, and right away, they are down that key. Another one of my keys is whether or not the candidate of the White House party is, like Obama was in 2008, charismatic. Hillary Clinton doesn't fit the bill.

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The keys have nothing to do with presidential approval polls or horse-race polls, with one exception, and that is to assess the possibility of a significant third-party campaign.

What about Donald Trump on the other side? He's not affiliated with the sitting party, but has his campaign been an enigma in terms of your ability to assess this election?

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Donald Trump has made this the most difficult election to assess since 1984. We have never before seen a candidate like Donald Trump, and Donald Trump may well break patterns of history that have held since 1860.

We've never before seen a candidate who's spent his life enriching himself at the expense of others. He's the first candidate in our history to be a serial fabricator, making up things as he goes along. Even when he tells the truth, such as, "Barack Obama really was born in the U.S.," he adds two lines, that Hillary Clinton started the birther movement, and that he finished it, even though when Barack Obama put out his birth certificate, he didn't believe it. We've never had a candidate before who not just once, but twice in a thinly disguised way, has incited violence against an opponent. We've never had a candidate before who's invited a hostile foreign power to meddle in American elections. We've never had a candidate before who's threatened to start a war by blowing ships out of the water in the Persian Gulf if they come too close to us. We've never had a candidate before who has embraced as a role model a murderous, hostile foreign dictator. Given all of these exceptions that Donald Trump represents, he may well shatter patterns of history that have held for more than 150 years, lose this election even if the historical circumstances favor it.

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We're a little bit less than seven weeks out from the election today. Who do you predict will win in November?

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Based on the 13 keys, it would predict a Donald Trump victory. Remember, six keys and you're out, and right now the Democrats are out — for sure — five keys.

Key 1 is the party mandate — how well they did in the midterms. They got crushed.

Key number 3 is, the sitting president is not running.

Key number 7, no major policy change in Obama's second term like the Affordable Care Act.

Key number 11, no major smashing foreign policy success.

And Key number 12, Hillary Clinton is not a Franklin Roosevelt.

One more key and the Democrats are down, and we have the Gary Johnson Key. One of my keys would be that the party in power gets a "false" if a third-party candidate is anticipated to get 5 percent of the vote or more. In his highest polling, Gary Johnson is at about 12 to 14 percent. My rule is that you cut it in half. That would mean that he gets six to seven, and that would be the sixth and final key against the Democrats.

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So very, very narrowly, the keys point to a Trump victory. But I would say, more to the point, they point to a generic Republican victory, because I believe that given the unprecedented nature of the Trump candidacy and Trump himself, he could defy all odds and lose even though the verdict of history is in his favor. So this would also suggest, you know, the possibility this election could go either way. Nobody should be complacent, no matter who you're for, you gotta get out and vote.

Do you think the fact that Trump is not a traditional Republican — certainly not an establishment Republican, from a rhetorical or policy perspective — contributes to that uncertainty over where he fits in with the standard methodology for evaluating the Keys?

I think the fact that he's a bit of a maverick, and nobody knows where he stands on policy, because he's constantly shifting. I defy anyone to say what his immigration policy is, what his policy is on banning Muslims, or whoever, from entering the United States, that's certainly a factor. But it's more his history in Trump University, the Trump Institute, his bankruptcies, the charitable foundation, of enriching himself at the expense of others, and all of the lies and dangerous things he's said in this campaign, that could make him a precedent-shattering candidate.

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It's interesting, I don't use the polls, as I've just explained, but the polls have very recently tightened. Clinton is less ahead than she was before, but it's not because Trump is rising, it's because Clinton is falling. He's still around 39 percent in the polls. You can't win if you can't crack 40 percent.

As people realize the choice is not Gary Johnson, the only choice is between Trump and Clinton, those Gary Johnson supporters may move away from Johnson and toward Clinton, particularly those millennials. And, you know, I've seen this movie before. My first vote was in 1968, when I was the equivalent of a millennial, and lots of my friends, very liberal, wouldn't vote for Hubert Humphrey because he was part of the Democratic establishment, and guess what? They elected Richard Nixon.

And, of course, as I have said for over 30 years, predictions are not endorsements. My prediction is based off a scientific system. It does not necessarily represent, in any way, shape or form, an Allan Lichtman or American University endorsement of any candidate. And of course, as a successful forecaster, I've predicted in almost equal measure both Republican and Democratic victories.