Despite its inherent bias against third-party candidates, the commission offers seemingly objective criteria for selecting participants. Two of these requirements are reasonable: Candidates on the stage must be constitutionally eligible for the job, and they must be on the ballot in enough states to theoretically win the Electoral College. These hurdles effectively exclude marginal candidates.

Mr. Johnson and his running mate Bill Weld—former governors of New Mexico and Massachusetts, respectively—will be on all 50 state ballots, and both are constitutionally eligible to hold the office. Only Mr. Trump and Mrs. Clinton can match that level of organization and support across the country.

But the third CPD requirement is grossly unfair. It requires participants to have support from 15% of the national electorate, as determined by five traditional telephone polls, most of which are landline-based. These old-fashioned surveys show the Johnson-Weld campaign at around 10%—likely because they underrepresent millennials, independents and people who use cell phones instead of landlines. These are all among the groups that most strongly support the Johnson-Weld ticket.