Johnson might also do well in Utah, where Mormons are "particularly rankled" by Trump's proposed temporary ban on Muslims entering the country, according to the Los Angeles Times. Johnson received 16 percent support in his best state poll there and won 13 percent in the most recent poll, which tied Clinton and Trump at 35 percent. In 2012, Mormon Mitt Romney took 72 percent of the vote in Utah.
Were Johnson to throw Arizona and Utah to Clinton, their combined 17 electoral votes would be enough to insulate her from losing Michigan's 16 electoral votes, if Trump's focus on the blue-leaning state pays off. Winning Arizona and Utah might allow her to lose every other swing state but Florida and still reach 270 electoral votes.
Johnson or Stein could also hurt Clinton in Colorado and New Mexico, though "some Democrats may be sufficiently risk-averse to want to be sure to avoid a repeat of 2000," said Phil Paolino, a political scientist who has studied past third-party campaigns. Nader's more recent candidacy influenced the outcome more than Ross Perot's, which took votes from both parties and did best in non-swing states. There is at least one scenario where the Green ticket could play a big spoiler role, as well: If Vermont Sen. Bernie Sanders accepts Jill Stein's offer to lead the party's ticket.
When there are three options, Johnson's national numbers are several points higher than Ralph Nader's at this point in 2000. Recent four-option polls have both Johnson and Stein meeting or exceeding Nader. They are drawing more money than third-party candidates did in 2012, and both are getting media attention. But they trail the strongest precedents in the current party system, George Wallace's 1968 run, John Anderson's 1980 campaign and Perot's runs in 1992 and 1996. All three polled in the double digits at this point, with Perot over 30 percent in June 1992. Johnson hovers around 8 percent and Stein is below 5 percent.
Stein's and Johnson's success likely reflects dissatisfaction with the major party candidates more than ideology, so their voters are highly vulnerable to negative partisanship.
Johnson told USA Today, "I understand that any third name — because of the disconnect or the polarization of Trump and Clinton — any third name would be registering." A Public Policy Polling survey had a giant meteor getting 13 percent.
Like meteors, Anderson, Wallace and Perot all dropped before Election Day. Only Perot ever led, and only Wallace won electoral votes. The American political system, with single-ballot, first-past-the-post elections, makes it extremely difficult for minor parties.
French political scientist Maurice Duverger observed why such rules favor two-party systems. The first reason is mechanical: "Third parties or independent candidates may win many votes nationwide, but gain a plurality of the vote in very few electoral units," wrote Paul R. Abramson, John H. Aldrich, Phil Paolino and David W. Rohde in "Third-Party and Independent Candidates in American Politics: Wallace, Anderson, and Perot." Illustrating this point, they wrote, "Perot won 18.9 percent of the popular vote without gaining a single electoral vote, because his support was distributed relatively evenly across the country."
This matches the current election: The polls-only model on 538, Nate Silver's data journalism site, projects Johnson getting 8 percent but gives him a 1 in 20 shot at even a single electoral vote.
The second reason is psychological: Supporters of third parties vote against their preferred candidates, because they worry that otherwise their last choices will win. In 2000, 6 in 10 self-described Nader supporters said they'd voted for Gore, presumably to stop George W. Bush from being elected. Another quarter voted for Bush. Anti-Trump conservatives in the Southwest may say now that they support Johnson, but come Election Day they will probably decide that they care much more about stopping Clinton.
Philip Edward Jones, a political scientist at the University of Delaware who has studied Duverger's law, said, "The decline in third-party support over time certainly fits the pattern of strategic voters figuring out only one of two candidates can win and settling for their second most-preferred candidate instead."
According to Lee Miringoff, director of the Marist Institute for Public Opinion, this may already be holding back Johnson and Stein. Clinton and Trump being so disliked might lead voters "to go for one of the two major party candidates to make sure the other one doesn't win." An earth-destroying meteor as a third-party candidate has the benefit of allowing neither to win, while Johnson may not.
Jones pointed out the importance of being treated seriously, as well. "If everyone (election officials/donors/journalists) expects one of the two major parties to win because of the electoral system, then of course they face extra challenges from these actors."
Candidates need 15 percent in five national polls to qualify for debates, which neither third party is achieving. They are not raising nearly as much money. In the latest reporting period, May, Johnson raised 6 percent as much as Trump and barely 1 cent for every dollar Clinton raised. Stein did worse, despite getting almost $100,000 in federal funds. May fundraising predates them clinching their nominations, so they may have since gone into higher gear, but, said Brian Doherty, senior editor at the libertarian Reason magazine, "It's not looking promising."
Unlike Anderson, neither fits as a moderate middle ground. Though Johnson is also a former Republican, he is now Libertarian, an ideology few Americans claim. Stein is to Clinton's left, a position with a constituency — consider Clinton challenger Sanders' success — but not enough of one — consider his eventual defeat.
Barry Burden, a political scientist at the University of Wisconsin-Madison who has studied third parties, said Johnson and Stein might hurt each other. He pointed to research showing that Nader had "two kinds of supporters: one was die-hard leftists on board with his platform, and the other set weren't particularly ideological but were dissatisfied with Bush and Gore." The larger the second set, the more Johnson and Stein will compete for voters.
Duverger's law has a flip side. In "expressive" voting, voters in lopsided districts can vote for third parties with no fear of affecting the outcome. Burden found that Nader did best in lopsided states, where his vote share "exceeded his polling, because people felt the freedom to vote however they wish," he said. "They thought a vote for Nader would send a message." While a landslide would minimize the role of third parties in swing states, it could be their best chance to win votes.
Paolino said this was most likely to help Johnson, because "voters who might otherwise vote for Trump to stop Clinton may reason that if Clinton is going to win anyway, there's more value for expressing disappointment (or disgust) for Trump by selecting Johnson."
Given the history of late-summer collapses, even maintaining current numbers would be a success for the Greens and Libertarians, and they are far from the 15 percent required to get on the debate stage or the double-digit point swing Johnson would need to take Utah. But he may still take enough of the vote to put the Southwest in play.
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