Donald Trump’s early weeks in office have been marked by a combination of the insurgent politics of his aide Stephen Bannon and stabs at conventional GOP governance. The paradox is that while Bannonism dominates the media and public debate, the Trump Presidency will rise or fall on whether he can pass a conservative reform agenda through Congress.

That’s the state of play as Mr. Trump prepares to deliver his first address to a joint session of Congress Tuesday evening. The Trump-Bannon light show—the immigration limits and...