Aside from their protest vote, they have another message: Don’t blame me for President Trump.

Much has been made of the fact that younger voters support third-party candidates at higher rates than older voters. In 2012, voters ages 18 to 29 voted for President Obama over Mitt Romney by 2-to-1. This year, there is a much larger share of undecided young voters heading into the election, and Mrs. Clinton’s support among likely voters under 30 drops by 10 points in four-way polls. But this line of reasoning ignores the bigger picture: Millennials still support Mrs. Clinton over Mr. Trump by a large margin, and at a higher rate than every older demographic. So maybe baby boomers and Gen-Xers should spend less time scolding their daughters for voting for Ms. Stein, and more time berating their dads for voting for Mr. Trump.

For some third-party voters, a logical paradox exists at the core of their decision-making process: The surer they are that Mrs. Clinton will win in November, the more confident they are voting for someone else. According to the polling firm YouGov, half of third-party voters believe Mrs. Clinton will win, compared with 15 percent who think Mr. Trump will.

Casting a protest vote is a tantalizing option for many: It gives you the veneer of moral purity while sacrificing nothing. In a normal election year, voters are often given the option to choose between two less-than-desirable options — would you like chicken or fish as your in-flight meal? In 2008, David Sedaris famously described an undecided voter contemplating the choice between the chicken and something I can’t mention here, mixed with ground glass. Voting for a third party this year is like being given the option between chicken and the plane crashing into the side of a mountain, but calmly asking for a banana split instead.

It’s still too early for pro-Clinton Democrats to wet the proverbial bed; third-party voters are a lot more pragmatic than some may seem (or sound). While the conventional wisdom about young third-party voters is that they are mindlessly throwing their votes away, many of the voters I talked to had thought long and hard about the efficacy of their vote, factoring in the partisanship of their home state. It’s easy for a Jill Stein voter in New York, or a Gary Johnson voter in Oklahoma, to rationalize his or her vote, which is likely to do little to swing their state’s electoral outcome.

“If I lived in a swing state, I would not hesitate to vote for Clinton over Trump, and I would be able to leave the polls with a relatively clear conscience,” said Caleb Hicks, a 27-year-old architect who plans to vote for Ms. Stein, the Green Party candidate, in garnet-red Louisiana. “I’m going to vote my conscience, and maybe pull the D.N.C. a little farther to the left next time around.”

While young voters like Mr. Johnson’s liberal stances on the war on drugs and criminal justice reform, his economic policies are straight from the trickle-down playbook. Mr. Johnson opposes Wall Street regulation and a minimum-wage increase, has promised to eliminate corporate and income taxes, opposes paid family leave, and supports the Keystone XL pipeline, the trans-Pacific Partnership, corporate political donations and a “free market approach to health care.” He believes in human-made climate change, but he doesn’t see it as a pressing threat because, as he said in 2011, “in billions of years, the sun is going to actually grow and encompass the earth.”

While Mr. Sanders did not win the Democratic nomination, he succeeded enormously in pushing Mrs. Clinton to the left on issues like college affordability and the minimum wage. But third-party voters on the left and the right agree that Mrs. Clinton can’t be trusted to keep her promises.