Think of it as the moment when Donald Trump truly learned to throw a (campaign) punch. It came about three weeks ago, amid the latest swirl of stories on the Clintons’ ethics. Hillary Clinton had recently blamed her private email server on Colin Powell. Judicial Watch had released more emails that showed the Clinton Foundation begging the State Department for special favors on behalf of its donors. Bill Clinton had floated laughable plans to reform the foundation.

The Trump campaign pounced. It began blasting out every new...