If ever a third-party presidential candidate was going to break out and shake the U.S. political system to its core, it seemed that 2016 could have been the year.

There's plenty of tinder for a prairie fire: a well-documented surge of populist anger and two of the most disliked Republican and Democratic nominees in history.

Yet as the campaign enters its final month, Libertarian Gary Johnson, the former governor of New Mexico, and Jill Stein of Massachusetts, the Green Party nominee, are fading in the polls. It appears the electorate is settling down to a familiar binary choice.

Johnson and Stein have been their own worst enemies in some ways. He has stumbled badly on questions about foreign affairs. She's a medical doctor who opposes childhood vaccinations.

They could still play a spoiler role in November if they siphon support in key states that otherwise would have gone to Hillary Clinton or Donald Trump.

But political scientists and strategists say that structural barriers - such as a lack of access to campaign money and an inability to draw media coverage - more than anything else determine whether a third-party candidate burrows into the national consciousness. It also helps if the two major parties have left a big ideological vacuum.

A wide difference

"When you look at the polls, yes, Clinton and Trump are deeply unpopular, but it's all relative," said James A. McCann, a political scientist at Purdue University who has studied third-party movements.

"The American electorate is fairly polarized along partisan lines," McCann said. "In order for a third party to succeed, there's got to be a groundswell of anger and frustration, and a significant number of people who dislike both major-party candidates equally - and, importantly, see no great difference between the two. This year, there is quite a bit of daylight between the two major-party candidates."

Trump's campaign has become a vehicle for voter anger, building a coalition of working-class Americans upset about jobs lost to foreign trade deals and conservative voters who want tighter control of immigration and an aggressive military approach to terrorism.

Vermont Sen. Bernie Sanders' challenge to Clinton in the Democratic primaries crystallized frustration with growing income inequality, the power of Wall Street banks, and corporate influence over politics. Clinton has adjusted her pitch left - including a proposal for tuition-free public college - and Sanders has endorsed and campaigned for her.

Missing the threshold

Meanwhile, neither Johnson nor Stein reached the 15 percent polling threshold required to be included in the three televised presidential debates, a failure that makes it harder than ever for them to get visibility.

Johnson is averaging about 7 percent in the Real Clear Politics aggregate of national polls; his high point in the website's survey was 9 percent, on Sept. 10. Stein is pulling down an average of 2.4 percent of the national popular vote, according to RCP. She crested at 4.8 percent on June 28.

This year, third-party backers are disproportionately young, according to polling, and the Clinton campaign, in particular, has been targeting millennials, arguing that they shouldn't waste a vote and help elect Trump. More than half of Johnson and Stein voters identified as independents, according to an Associated Press-GfK poll released Oct. 1.

As the election draws closer, some third-party backers are changing their minds. In a Quinnipiac University poll released Friday, for instance, Clinton had the support of 48 percent of voters aged 18 to 34, the coveted millennials. In the university's mid-September poll, she had 31 percent of their backing.

Johnson had 11 percent support among millennials, down from 29 percent in the earlier survey. Stein was at 9 percent, down from 15 percent.

Quinnipiac also found a big movement of independent voters in Clinton's direction in Friday's poll, following Trump's perceived loss of the first debate Sept. 26 and continuing controversy over his treatment of women.

"It's exactly what Clinton wanted to see happen," said Tim Malloy, assistant director of the Quinnipiac Poll. "We're getting down to brass tacks, the existential questions. People are thinking hard about the election, and they're making their choices."

The United States has had a two-party system since the early 19th century, though the shapes and identities of the two parties have changed over time, often due to challenges from third-party movements. The most successful of them arose because of splits in the major parties amid economic or social turmoil.

In 1912, former President Theodore Roosevelt bolted the GOP to run on the Progressive Party ticket. Despite his stature, he won only six states, finishing well behind Democrat Woodrow Wilson, through Roosevelt did win 27 percent of the popular vote. Both parties adopted progressive ideas on government regulation and civil-service reforms.

In 1968, the segregationist Alabama Gov. George Wallace ran against the party's deep embrace of civil rights, taking advantage of a backlash among white Southern Democrats (and some in the North as well). He won five Southern states.

Republican Richard Nixon, running as the law-and-order candidate, adapted some of Wallace's positions and launched a strategy to appeal to Southern white voters that turned the reliably Democratic region into a Republican stronghold within a generation.

In 1992, the economy was mired in a recession. Voters were upset with President George H.W. Bush for breaking his campaign promise not to raise taxes; and many voters were eager for a change after 12 years of Republican rule in the White House - but Democrat Bill Clinton was dogged by scandal.

Perot's moment

A Texas billionaire, Ross Perot, won 19 percent of the popular vote that fall. He made it to the debate stage, and his opposition to free-trade agreements previewed Trump's. Perot also railed against the federal budget deficit, and reducing spending became a priority for President Bill Clinton and congressional Republicans.

This year, Johnson and Stein have caught the attention of voters because of antipathy toward Clinton and Trump, not because of their advocacy of particular issues.

Margaret Bonnem, a stay-at-home mother in Colliersville, Tenn., previously supported Stein but told the AP pollsters she now realizes that "a third-party candidate can't really do anything but pull votes away" from major-party candidates who can actually win.

"I can't vote for Trump, and I don't want him to benefit from me voting for someone else," said Bonnem, 54, who has decided on Clinton. "So I'll end up voting for someone I don't fully trust."