Could a third-party candidate appear on stage at the Sept. 26 presidential debate? That decision will be made by the small group of prominent Democrats and Republicans who control the Commission on Presidential Debates (CPD). Given the American public’s widespread dissatisfaction with Donald Trump and Hillary Clinton, the commission ought to let the Libertarian nominee Gary Johnson make his case.

The ritual of presidential debates overseen by the CPD may seem like a long-enshrined tradition, but it isn’t. Established less...