Nonetheless, the similarities between the two figures are remarkable. Both came to Washington with no previous political experience. When Mr. Smith uncovers graft on the part of the senior senator from his state, the political machinery revs up against him full-bore. Shining a lantern on political corruption, Mr. Smith is himself accused of corrupt dealing—sound familiar?—by critics who say Mr. Smith’s legislation to start a boys’ camp will line the senator’s own pockets. The allegation is false, but that doesn’t stop a throng of senators from hastily calling for Mr. Smith’s expulsion from the Senate. Nancy Pelosi may want to view Capra’s film again to see how well that worked out.

Mr. Trump is a born disrupter. Mr. Smith has disrupter status thrust upon him but comes to relish it no less. Mr. Trump rails endlessly about the press’s distortion of his record. When Mr. Smith’s young Boy Rangers ride bicycles to his rescue, their newspapers—the only ones not on the payroll of corrupt tycoons and politicians—are confiscated. The radio stations are also all on the take. The media controls the narrative; the public is kept in the dark.

Reminiscent of Mr. Trump’s frequent “I’m not a politician” disclaimer, an all but broken, filibuster-weary Jefferson Smith acknowledges his outsider status to his fellow senators in a climactic scene: “A guy like me should never be allowed to get in here in the first place.” But he was. And despite all efforts to stop him, so was Mr. Trump.