One of the great unknowns of the current presidential election cycle—a cycle without recent precedent in its unusual twists and turns—is whether the fall campaign will see a convergence of the populist left and the populist right. Could a chunk of Bernie Sanders’ anti-establishment base really vote for GOP outsider Donald Trump in November? Could it be enough to swing the election in Trump’s favor?

Many experts scoff at the idea. “Not a chance,” former presidential adviser Jim Messina said on Monday. Writing for Politico Magazine, the always perceptive Bill Scher was also skeptical. The “few areas of ideological overlap don’t come close to outweighing the long list of issues where Sanders and Trump are practically opposites,” he wrote.

But here’s the problem with that theory of the case: History shows us that when disenchantment with establishment politics and institutions is high, as it is today, voters don’t always vote along ideological lines.

In 1968 and 1980, insurgent liberal challengers—Eugene McCarthy and Ted Kennedy—captured a popular wave of anti-establishment sentiment but failed to win their party’s nomination. In November, many of their supporters veered sharply to the right, voting for candidates who didn’t necessarily share their political views but who served as a convenient outlet for the expression of their broader frustrations. In both cases, this block of Democratic defectors helped deliver the election to the Republican Party. 

It’s a startling historical reminder for Hillary Clinton and her backers as we head into the summer: When voters see America as a nation in decline, and when they attribute the fraying of its economic and social fabric to elite conspiracy, they sometimes cast their votes based on emotion rather than logic. ...