My model shows Donald Trump has an 87 percent chance of beating Hillary Clinton

To be sure, Donald Trump, is a long shot in betting markets to win in November. PredictIt, a popular legal wagering website, gives Hillary Clinton a 66 percent chance to win the presidency. She has consistently led Trump in that market for three months, as well as in the Iowa Electronic Markets. And Trump has trailed Clinton — with rare exceptions — in the poll averages by RealClearPolitics and The Huffington Post.

So how can a reasonable person predict that Trump will be the next president?

For starters, pre-election polls have selected the wrong candidate many times. Who can forget Tom Dewey defeating Harry Truman in 1948 polls — until he didn’t? Or Michael Dukakis leading George H.W. Bush in 1988 by 17 points this time of year? Or Mitt Romney edging Barack Obama in the final Gallup poll four years ago?

My advice: Beware of pollsters bearing forecasts, especially anyone trying to peek into the future, especially those with money to bet.

Some 20 years ago, I constructed a formula, The Primary Model, that has predicted the winner of the popular vote in all five presidential elections since it was introduced. It is based on elections dating to 1912. The formula was wrong only once: The 1960 election. That one hurt because John F. Kennedy was my preferred candidate.

The Primary Model consists of two ingredients: The swing of the electoral pendulum, and the outcomes of primaries.

You can see the pendulum work with the naked eye. After two terms in office, the presidential party in power loses more often than not. In fact, over the past 65 years, it managed to win a third term only once. In 1988, President George H.W. Bush extended Ronald Reagan’s presidency by one more term. Reagan made this possible by winning re-election by a bigger margin than when he first got elected. That spells continuity, a desire for more of the same.

President Barack Obama has not left such a legacy for a Democratic successor. He did worse in his re-election victory over Mitt Romney in 2012 than when he beat John McCain in 2008. That spells, “It’s Time for a Change!” The pendulum points to the GOP in 2016, no matter whether the candidate was named Trump, Ted Cruz, Marco Rubio, John Kasich or whoever.

Now add the outcomes of presidential primaries. Although some experts claim primary votes have no bearing on general elections, the fact is that primaries prove uncanny in forecasting the winner in November. Take the first election with a significant number of primaries, in 1912. In November that year, Woodrow Wilson, the winner in Democratic primaries, defeated William Howard Taft, the loser in Republican primaries; Taft was renominated since most states then did not use primaries. In general, the party with the stronger primary candidate wins the general election.

This year, Trump has wound up as the stronger of the two presidential nominees. He won many more primaries than did Clinton. In fact, this was apparent as early as early March. Trump handily won the first two primaries, New Hampshire and South Carolina, while Clinton badly lost New Hampshire to Sen. Bernie Sanders before beating him in South Carolina.

The Primary Model predicts that Trump will defeat Clinton with 87 percent certainty. He is the candidate of change. When voters demand change, they are willing to overlook many foibles of the change candidate. At the same time, the candidate who touts experience will get more intense scrutiny for any missteps and suspicions of misconduct of the record of experience.

Trump may be lucky to have picked an election in which change trumps experience and experience may prove to be a mixed blessing.

Helmut Norpoth is the director of undergraduate studies and political science professor at Stony Brook University.