Search Google for Donald Trump buffoon and you will get more than 50,000 hits in return. As that suggests, the tendency to dismiss Mr. Trump, the billionaire populist Republican presidential candidate as exactly that—a buffoon—is pretty widespread.

It’s also a mistake. Mr. Trump is important for two reasons: first for the damage he can do to the Republican Party, and second for the useful lesson he can teach that same party. The potential damage comes largely in the harm he can do—indeed, already may have done—to Republicans’ crucial mission of building better bridges to Hispanics. The lesson comes by way of illustrating the depths of populist anger running through sectors of the GOP right now…

Mr. Trump’s burst is a sign that there is a populist streak at the base of the party. This is in part the natural result of the GOP’s expansion in the past three decades to include more working-class voters.

Today, many of them feel economically threatened and marginalized by cultural change. Some cite a decline in moral values as the most alarming trend in the country. They aren’t the genteel patricians of Republican stereotype, but they are Republicans nonetheless.