Donald Trump at a rally in Aston, Pennsylvania. REUTERS/Jonathan Ernst A professor who has accurately predicted the outcome of every presidential election since 1984 told The Washington Post that 2016 was the hardest election to predict yet.

But he has come to a conclusion about who is most likely to win — Republican nominee Donald Trump.

Professor Allan Lichtman, who wrote the book "Predicting the Next President: The Keys to the White House," uses a series of true/false statements to determine his predictions. He considers things like incumbency, the economy, social unrest, scandals, and charisma to figure out which way the election is likely to go.

Lichtman told The Post how the system works:

"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."

Still, Trump is such an unusual candidate that he might break the American political mold that has held for decades.

"We've never before seen a candidate who's spent his life enriching himself at the expense of others," Lichtman said. "He's the first candidate in our history to be a serial fabricator, making up things as he goes along."

He noted that Trump has also taken other questionable steps, like embracing Russian President Vladimir Putin, inviting Russian hackers to meddle in the US election, and threatening to blow Iranian ships out of the water if they came too close to US vessels.

"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," Lichtman said.

It looks like a narrow victory for Trump, Lichtman said, and the key is Libertarian Party candidate Gary Johnson, who is polling in double-digits now. But that might shift by Election Day.

"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," Lichtman said.

Trump has been surging lately in the polls, but last month his Democratic challenger, Hillary Clinton, was far ahead.

Other less fickle measures of predicting the election put Clinton out in front. An Electoral College map released last month from the University of Virginia Center for Politics projected Clinton to win the election by a landslide.