Libertarian Gary Johnson and the Green Party's Jill Stein are offering voters an alternative to Trump and Clinton. Here's everything you need to know:

Who is Gary Johnson?

The Libertarian candidate is a businessman who served two terms as the popular Republican governor of New Mexico. Johnson, 63, a fitness fanatic who has climbed Mount Everest, is also the former CEO of a legal cannabis company and has long supported marijuana legalization — though he has pledged not to use the drug in the White House if he's elected. His running mate is another former Republican governor from a blue state: Bill Weld, of Massachusetts. While in office, both men pursued the libertarian principles of small government and less regulation — cutting taxes and vetoing spending programs. But both say they have been driven from the GOP by religious conservatives who want to use the government to control other people, and by Tea Party extremists who want to wreck government altogether. "I've always believed that Republicans are mostly about smaller government," says Johnson. "But of late? Not at all." Instead, he says, the far right is asking, "What's to crash? What's to ruin? What's to spoil?"

What are their policies?

Johnson and Weld are campaigning on a traditional libertarian ticket of small government and individual freedom. As fiscal conservatives, they support a single consumption tax and want to scrap the Dodd-Frank law regulating Wall Street. And while Johnson drew boos among hard-core delegates at the Libertarian Party National Convention for supporting driver's licenses, he and Weld generally conform to the libertarian belief that government works best by staying out of people's lives. They favor legalizing marijuana and oppose gun restrictions. They initially both opposed mandatory vaccinations (though Johnson has since changed his position), and support the right to abortion and gay marriage. "We want the government out of your pocketbook," says Weld, "and out of your bedroom."

What about Stein?

A Harvard-trained physician turned liberal activist, the Green Party nominee is selling herself as the progressive "Plan B" for disaffected Bernie Sanders supporters — many of whom have pledged never to vote for "establishment shill" Hillary Clinton. "I almost feel like a social worker, being out there talking to the Bernie supporters," says Stein, 66. "They are brokenhearted. They feel really abused." Stein echoes many of Sanders' policies: She supports a $15-per-hour federal minimum wage, a "wartime-scale mobilization" to defeat climate change, a single-payer health-care system, and the abolition of all student debt. But she goes much further to the left, calling for unilateral nuclear disarmament, a 50 percent cut to the military budget, the elimination of all fossil fuels by 2030, and paying reparations for slavery. Although Stein is a doctor, she has expressed concerns about the safety of vaccinations and Wi-Fi. Her running mate, human rights activist Ajamu Baraka, once called President Obama an Uncle Tom who's betrayed African-Americans.

Could these candidates win?

The chances of that are close to zero. But the two main-party nominees, Democrat Hillary Clinton and Republican Donald Trump, are the least liked candidates in the modern political era. Four out of 10 voters think neither would make a good president — leaving the door wide open for Johnson and Stein to attract a significant protest vote. Johnson has climbed to over 10 percent in some polls, and in a four-way matchup against Trump and Clinton, he and Stein together collect up to 14 percent of registered voters. That's enough to affect whether Trump or Clinton wins in some critical swing states.

Which candidate will they hurt more?

That's not clear. Trump will lose support to Johnson among Republican "NeverTrumpers" who don't view the bombastic businessman as a true conservative, and who are attracted to Johnson's fiscal conservatism and pro–gun rights record. Clinton, though, could get hit on two fronts. Many hard-core "Bernie-or-Bust Democrats" are pledging their allegiance to "Jill, not Hill." But Hillary could also lose votes to Johnson, whose support for marijuana legalization and a rollback of government surveillance appeals to young voters. That has led many Democrats to warn that Johnson and Stein could jointly become the Ralph Nader of the 2016 election.

What do they mean?

In 2000, Nader, the Green Party candidate, pulled thousands of votes in Florida away from Democrat Al Gore — a state George W. Bush won by just 537 votes. That gave the election to Bush, even though he lost the overall popular vote to Gore. This year, if the election proves tighter than polls currently indicate, it's possible that Johnson and Stein could cost Clinton enough votes in a key battleground state or two to alter the outcome of the election. Johnson embraces the idea of being the "spoiler" candidate. "I will lose no sleep if that's the label given to me," he says. "This is a party that needs crashing." Stein also rejects the idea that voters must choose "the lesser evil," arguing that Clinton is not a true progressive. "The American people are clamoring for another choice," she says.

The debate stakes

Both Johnson and Stein are desperate to make it into the first presidential debate, on Sept. 26. But they face a steep climb. A candidate needs to hit 15 percent in five national polls to make it onto the stage, according to debate commission rules. Johnson is polling at an average of 8.8 percent, while Stein is at 3.4 percent. The two candidates face a classic catch-22: They won't be included until they prove they are competitive in the polls, but can't compete without the attention and name recognition a prime-time presidential debate would bring. Johnson and Stein are suing the commission for the right to participate. If they were part of the debates, the race might be very different. "For people to choose Gary Johnson, they need to know that he's running," says political operative Roger Stone, an informal adviser to Trump. In 2012, Stone backed and provided advice to Johnson's first presidential run as a Libertarian, when he won just 1 percent of the vote in the race against President Obama and Mitt Romney. "There was another choice," Stone says, "it's just that nobody knew about it."

Editor's note: This article originally mischaracterized Johnson's position on mandatory vaccinations. It has since been corrected. We regret the error.