“I’ll get to know Gary Johnson better and see if he’s someone who I could end up voting for,” Mr. Romney told CNN in June, adding that “Marijuana makes people stupid.” (Yes, Governor, that is kind of the point.)

If there were a time for Mr. Johnson to gain traction, that time would be now. Third-party candidates tend to poll the best in the summer leading up to a presidential election, before voters begin to seriously consider the ramifications of “wasting” their vote. In June 1992, the Texas billionaire Ross Perot led both George H. W. Bush and Bill Clinton in the polls.

“In the summer before an election, that’s generally when a third-party candidate will look their best,” said Seth Masket, a political scientist at the University of Denver. “Fall campaigns tend to remind Republicans what they hate about Democrats, and Democrats what they hate about Republicans.”

Mr. Perot ended up earning nearly 19 percent of the popular vote in 1992, while Ralph Nader drew 3 percent in 2000. While Mr. Perot’s 1992 candidacy is treated as a quirky blip in American electoral history, Mr. Nader’s 2000 run left many center-left voters with liberal guilt for ushering in a second Bush era. Since the election, studies have found that the race in Florida was indeed close enough for Mr. Nader to swing the election in Mr. Bush’s favor (though that was not his intended effect).

“Voters who were of age during the 2000 controversy were severely enough traumatized to shy away from voting for third-party candidates,” said Daniel Franklin, a political scientist at Georgia State University. “But for younger voters in particular, third-party candidates present a legitimate option. Nevertheless, third-party candidates have no chance of winning the election.”

Despite gaining some traction from the Bernie-or-Bust crowd, Dr. Stein’s Green Party is not positioned to do well this fall. Unlike Mr. Nader, who had gained some celebrity as a consumer-rights advocate before running for president, Dr. Stein remains relatively unknown. And for a doctor, she holds some unorthodox views on vaccines and the health effects of wireless internet.