Democratic presidential candidates clashed in a Tuesday night debate over how they would change course from President Trump’s protectionist trade agenda in fiery exchanges that underscored the depth of the rift within the party over economic policy.

The strength of the United States economy, which has enjoyed steady growth and historically low unemployment in recent years, represents Mr. Trump’s greatest asset going into his re-election campaign and a conundrum for Democrats. On Tuesday night, the candidates strained to paint what Mr. Trump touts as an economic miracle as a mirage, pointing to widening income inequality and trade policies that they argue have done American workers more harm than good.

For two and a half hours, moderate and progressive candidates grappled with whether pragmatism or idealism was the best formula for defeating Mr. Trump next year. It was the subject of trade, which has become the centerpiece of Mr. Trump’s agenda, that most animated the candidates. They argued over whether to revisit Obama-era multilateralism or double down on Mr. Trump’s brand of economic isolationism, which has upended how both Republicans and Democrats think about international commerce. […]