In Tennessee’s critical Senate race, it’s the Democrat running as a free trader. Phil Bredesen, a former governor, poses in a whiskey distillery in a recent ad and slams President Donald Trump’s tariffs: “They hurt our auto industry, our farmers, and Tennessee exports like Jack Daniel’s.”

Since the 1940s, Republicans have branded themselves the party free trade, while Democrats—especially rank and file officials and congressmen—have more often been the party of protection. Those labels need updating. Mr. Trump’s imposition...