WASHINGTON — The United States is in the midst of one of the longest economic expansions in its history. Even American factories have lately added hundreds of thousands of jobs.

Meanwhile, the rest of the world is stumbling. Even China.

Yet at perhaps the least likely moment in the last several decades, misgivings about globalization are playing a starring role in the presidential election. Why now?

Anger about unbalanced trade has helped to fuel the rise of Donald J. Trump, the Republican front-runner, and the success of Senator Bernie Sanders of Vermont in his bid for the Democratic nomination. The manifest anger also has pushed their principal rivals, Republican Senator Ted Cruz and the Democratic front-runner, Hillary Clinton, to toughen their own trade rhetoric.



It is a situation that has surprised many experts because polls show voters’ concern about the overall health of the American economy has declined significantly in recent years.