The Libertarians believe health care should be handled by the private sector — goodbye, veterans’ hospitals — and would end Social Security. The party is devoted to free trade and dislikes any international trade treaties, including those that protect workers. The party is against all forms of foreign aid and military assistance, and a direct attack on the United States is the only instance in which Libertarians favor a military response.

Mr. Johnson draws less than 10 percent in the polls that include the third-party candidates. If pollsters included him in their surveys every time, he says, he’d be doing better, though not as well as, say, Mickey Mouse. “Mickey would be at 30 percent,” he said on PBS on Thursday.

Ms. Stein, the Green Party candidate, is a Harvard-trained doctor and environmental activist. She is running on a “Power to the People Plan,” which she says “creates deep system change, moving from the greed and exploitation of corporate capitalism to a human-centered economy.” She wants the federal government to “buy” all existing student loan debt, at a cost of about $1.3 trillion. That’s about one-third of what the entire federal government spent in 2015 — on everything. On top of that, she would also throw in free public university education. These proposals are so off-the-charts unaffordable that they would never pass any Congress, regardless of which political party held a majority.

Ms. Stein proposes to slash military spending in half and close every United States military base on foreign soil — some 700, including bases used in the war on terrorism, as bulwarks against Russian and Chinese aggression and as staging areas for humanitarian assistance to victims of war and disaster. That would send an isolationist message to our allies that would rival Mr. Trump’s.

Ms. Stein polls at around 3 percent. The Green Party is the party of Ralph Nader, who in 2000 drained votes away from Al Gore. Here’s what Ms. Stein says about that: “The Supreme Court stopped the vote count, and Gore was just rolled over. … Nobody else, in fact, created that loss.”