Gary Johnson and Jill Stein have a difficult task—though this election year it might be easier than most. The trick for third parties in American politics is convincing voters that they aren’t “wasting” ballots by supporting the Libertarian or Green Party candidate, since neither will make it to the White House.

But the unpopularity of Donald Trump and Hillary Clinton has provided the Libertarians and Greens with a new argument: Political parties that meet defined benchmarks on Election Day are given certain advantages under state and federal law. If Mr. Johnson and Ms. Stein manage to secure 5% of the popular vote—plausible given current polling—their parties will reap significant benefits.

The biggest might be automatic placement on the ballot. In many states minor parties are required to gather a certain number of voters’ signatures, sometimes tens of thousands, to get their candidates listed. That means circulating petitions, with a “cost per signature” of between $1 and $5, depending on the state. The Libertarian and Green parties have spent millions of dollars this year simply to put their candidates on the ballot. Parties that perform well on Election Day are often allowed to skip this onerous process.

Consider Minnesota. The Libertarian and Green parties needed to secure 2,000 signatures from registered voters to secure ballot access. That usually means collecting more than 3,000 signatures in case some of them are found to be invalid.

There are costs to the process besides money. The Green Party started collecting signatures in June, before Ms. Stein had picked her vice-presidential candidate. The petitions in Minnesota named a stand-in, Howie Hawkins. But in August Ms. Stein selected Ajamu Baraka instead. It was too late. The Minnesota ballot in November will list Stein-Hawkins instead of Stein-Baraka.