The leftward shift in the Democratic Party virtually assures that the nominee will be branded a socialist by Mr. Trump. That nominee would be wise to point to the military policies of countries like Sweden, Norway and Denmark, which fund their military in far more sustainable ways than the United States, with military spending that doesn’t carve trillions into the national deficit and with conscription laws that ensure all citizens share equally in the burdens of war.

Mr. Trump has the advantage of being the incumbent and will lean on his foreign policy achievements, which in some cases are significant. They are, however, strategically incoherent: a shaky peace deal with the Taliban; the execution of an Iranian general, Qassim Suleimani; a U.S. troop withdrawal from Syria; nuclear threats against North Korea followed by photo ops with Kim Jong-un.

Just as he’s kept his adversaries guessing what he’ll do next, the president has done the same to the American people, who largely understand that the world is a dangerous place and that we are being led through it by a president known for uniquely low impulse control. No demographic feels this more acutely than military families, for whom Mr. Trump’s drastic foreign policy shifts often come with deployment orders attached.

Already, in the wake of the Suleimani strike, a bipartisan group of lawmakers has called for renewed scrutiny on presidential war powers. A Democratic nominee who continues to touch upon the importance of heightened congressional oversight of presidential war powers would find fertile ground among military members who have been deployed to combat zones as far-flung as Syria and the Horn of Africa under an Authorization for the Use of Military Force signed nearly two decades ago by President George W. Bush.

The backslapping solidarity that Mr. Trump will display with the military on the campaign trail — whether it’s reliving Conan the dog’s exploits when chasing down Abu Bakr al-Baghdadi or hobnobbing with cable news hosts wearing shotgun blasts of military lapel pins above fuchsia pocket squares — can be undermined by a candidate who speaks to these rarely discussed, and often verboten, issues.