Donald Trump is not the only one who can say, "I told you so."



A few college professors stood up to a tidal wave of prevailing thought to separately predict for months that the New York billionaire and reality TV star would pull off one of the biggest shockers in American political history.



A political scientist at Stony Brook University based his prediction on a formula using primary results and his "pendulum of change" theory. A Yale professor tied his pick to economic factors. And a history professor at American University relied on a formula he developed in the 1980s.



"I had a lot of people saying, 'This isn't going to work. You're going to fall on your face.' I got emails berating me for being an idiot and irresponsible," says Helmut Norpoth, a longtime political science professor at Stony Brook.



Norpoth has successfully predicted every presidential winner since developing the formula for the 1996 presidential race. He also used his formula to review every presidential election since 1912, and found the indicators would have accurately predicted the outcome every time except 1960.



He acknowledged he harbored some private doubts as this year's contentious campaign raged on, he never wavered publicly, even doubling down on the pick after Trump's explosive "Access Hollywood" video came out.



"I put up a good face," he says. "But deep down I got a little uneasy, I can't deny that."



Norpoth says his model is based, in part, on the idea that candidates who excel in primaries tend to do better in the general election. He says Trump's victory and Clinton's defeat at the hands of Sen. Bernie Sanders in the New Hampshire primary was a key indicator for him of the Republican's strength.



He also factored in that Barack Obama had held the White House for two terms and estimated the pendulum could swing Republican. The only time in the past 30 years that either party won three straight presidential elections was in 1988, when Republican George H.W. Bush defeated Democrat Michael Dukakis.



Also successfully predicting a Republican victory was Yale University professor Ray Fair. He used economic factors - GDP growth and inflation - and not the vagaries of either candidate's personality in his prognostication.



American University professor Alan Lichtman said he uses 13 factors to assess the performance of the party in power to determine whether it will keep the White House. He examined presidential elections from 1860 to 1980 to establish the factors to develop his study. He said the factors this year narrowly showed a Republican would prevail.



Lichtman's success comes with an asterisk. He has accurately calculated the winner of the popular vote in every election since 1984, but Al Gore actually lost in the electoral college in 2000, as it appears has happened to Clinton this year.



"I feel vindicated in a sense, but I had issued a qualification because in Donald Trump you had a history-smashing candidate," Lichtman says.