House Democrats can begin using their “subpoena cannon” in January to investigate and harass the White House, hoping to convince the nation of President Trump’s unfitness for office—though it is hard to imagine what they could possibly unearth that would be more damaging than what is already known. Alternately, they could try to abolish U.S. Immigration and Customs Enforcement and replace private health insurance with government coverage, convincing not one new voter (or the Senate) to join them. A more effective strategy would be for House Democrats to take Mr. Trump’s populist campaign rhetoric seriously and seek to…