Current polls show Gary Johnson poised to secure 7.1 percent of the vote on Election Day, far better than his 2012 performance — or any third-party candidate of the 21st century. Jill Stein is far behind, at 2.4 percent, but that’s still about on par with what Ralph Nader did in 2000 and well ahead of subsequent Green Party bids in 2004, 2008, and 2012.

A strong third-party showing is about what you’d expect given two unusually unpopular major party candidates. But there’s good reason to doubt that the Libertarian and Green vote totals will really be that high. One reason is that as my colleague Tara Golshan has written, third parties tend to fade in the polls as Election Day approaches — something that’s happened to both Johnson and Stein last week — but more fundamentally the Roper Center’s historical analysis suggests that third-party campaigns tend to underperform their final poll numbers.

In the November 2000 Gallup poll, Nader was at 4 percent and Pat Buchanan 1 percent. In the end, Nader got 2.74 percent and Buchanan got 0.43 percent.

In the November 2004 Gallup poll, Nader was at 1 percent and he got 0.38 percent.

In the November 2008 CNN/ORC poll, Nader was at 5 percent and he got 0.56 percent. Bob Barr had 2 percent in this same poll, and he got 0.4 percent.

The November 2012 CNN/ORC poll showed 3 percent for Johnson and 1 percent for Stein; he ended up with 0.99 percent and she got 0.36 percent.

One reason polls tend to overstate support for Green and Libertarian candidates is that we are talking about a group of voters — disproportionately younger people, disproportionately with weaker ties to the political system — who are more challenging to poll under modern-day conditions.

Since these third-party votes typically aren’t relevant to the outcome, there aren’t any particularly strong incentives in place to work hard at improving. If you overstated the level of support for the Republican or Democratic candidate by a factor of three, you’d have committed a humiliating mistake that needs fixing. But nobody really cares about predicting the Green vote precisely.

But beyond modeling issues, third-party candidates simply face a fairly profound turnout problem.

They are starting, almost by definition, with a challenging base of supporters to turn out — people who feel disconnected from the political system and unserved by major political actors. On top of that, the fact that third-party campaign messages, by definition, can’t really be based around emphasizing the weighty moral stakes of the campaign or the profound civic obligation to play a constructive role in shaping the outcome. Last and by no means least, third-party campaigns don’t have party turnout operations behind them, down-ballot candidates shaking the trees for voters, or the visibility of a major campaign.

At the end of the day, if your main interest is in protesting the system, the passive protest of staying home is easier and perennially more popular than the active protest of showing up and voting third party.

Given the lack of field staff associated with third-party campaigns, actually getting to the polls and voting requires a great deal of personal commitment. And there are serious reasons to doubt that third-party voters actually are all that committed to what they are telling pollsters. When you ask about Jill Stein, for example, about 3 percent of people say they’ll vote for her. But if you ask instead about Monica Moorehead, of an obscure Maoist political party that’s not on the ballot in most states, she polls just as well as Stein:

Monica Moorehead (of the Workers World Party) does as well as Jill Stein when read in a ballot test https://t.co/FtEVbl7qqR pic.twitter.com/7b7IDCuFhM — David Shor (@davidshor) October 7, 2016

The Prohibition Party’s James Hedges doesn’t poll quite as well as Gary Johnson, but it’s still the case that 4 percent would be a staggering strong performance for a third-party candidate if it happened. But it’s not going to happen.

In other words, whatever it is that inspires people to tell pollsters they are voting third party, it in most cases isn’t profound personal commitment to the candidate, the cause, or the party. That means these third-party voters tend to disappear in the voting booth — either ultimately aligning themselves with a major party or, more likely, not showing up at all.

Watch: The bad map we see every presidential election