Is support for third-party candidates fading away?

While Donald Trump and Hillary Clinton went back and forth on national television for the first time last month, Libertarian candidate Gary Johnson sat in an office 30-something miles away, watching closely with social media at the ready. 

When the Republican and Democrat take the stage again Sunday night for the second debate, Mr. Johnson and Green Party candidate Jill Stein will once again be noticeably absent. In an election year with two unprecedentedly disliked major party candidates, many have called for the inclusion of their third-party challengers. But support for Johnson and Ms. Stein, who appeared to have a greater chance than most other third-party candidates, has slowed and waned in recent weeks as the two major party candidates quite literally take center stage.

At the moment, Johnson is pulling down about 6.5 percent of the national popular vote in RealClearPolitics's aggregate of national polls, a slight drop from his high of 9 percent one month ago. Stein is averaging 2.3 percent, roughly half of her high of 4.8 percent in June. 

Much of the support for third party candidates in this election has come from Millennial voters – but a recent Quinnipiac University poll shows that this demographic has begun to shift its support to Hillary Clinton. In a mid-September poll, Johnson had the support of 29 percent of Millennials; in the poll published Friday, he was down to 11 percent. Similarly, support for Stein had dropped from 15 percent to 9 percent.

It could be argued that part of the shift may be attributed to the candidates themselves. Gary Johnson in the past month made headlines for two uncomfortable on-air moments. First, when asked about the Syrian city of Aleppo in an interview, he asked, "What is Aleppo?" Then, when asked to name a foreign leader he admires, he was unable at first to recall the name of former Mexican president Vicente Fox. 

However, as David Iaconangelo reported for The Christian Science Monitor, the incidents didn't appear to hurt Johnson in the polls – on the contrary, his peak support of 9 percent came just after the "Aleppo moment" – suggesting that the recent drop may be due in larger part to the two major party candidates.

The latest Quinnipiac poll, which follows the first debate and ongoing controversy over Trump's statements about women, shows support for Clinton skyrocketing from 31 percent to 48 percent among Millennials in just a matter of weeks.

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

While Trump comes under fire for his treatment of women, Clinton has also been hitting the campaign trail with Bernie Sanders, delivering speeches on college affordability, in an effort to win over disenchanted Millennial voters. 

But while these factors may have contributed to the shift away from third-party support, political scientists say that a decline in support for third-party candidates is keeping with the trends of past elections. Lawrence Jacobs, a professor of political science at the University of Minnesota, told the Monitor in July that support for Stein and Johnson would likely wane as the election draws closer.

"[T]here's a tendency to see the third party threat as larger in the beginning of a general election cycle than it turns out to be," he said.