Fifteen percent is the magic number Libertarian presidential candidate and former governor Gary Johnson needs to reach to earn his voice in the 2016 election.

By capturing the support of 15 percent of voters in national public opinion polls, Johnson could join the major party's presidential candidates on the primetime debate stage. With both Democratic and Republican presidential candidates disliked at historic levels and a rising share of political independents frustrated with the two major parties, this is the year a third-party candidate like Johnson has a realistic chance of getting onto the debate stage.

What would it take?

Johnson would need to receive an invitation to participate in the debates from the Commission on Presidential debates (CPD), a private, non-partisan, 501(c)(3) organization that has sponsored the general election presidential debates since 1988. The CPD is not a government entity, nor does it receive government funding. But it is a creation of the two major parties, co-chaired at its inception by both the Republican and Democratic parties' national chairman.

The CPD establishes objective eligibility criteria in advance—compliant with regulations enforced by the Federal Election Commission—that all candidates must meet, including the Democratic and Republican candidates, to be invited to participate in the debate.

The CPD's 2016 debate eligibility criteria, announced last year, are as follows:

Besides being constitutionally eligible, the candidate would need to appear on a "sufficient number of state ballots to have a mathematical chance of winning a majority vote in the Electoral College." Currently the Libertarian Party has ballot access in 32 states and is confident about getting on the remaining 18.

Next, Johnson would need to have the support of at least 15 percent of "the national electorate" as determined by the average of five selected national public opinion polling organizations' most recently publicly reported results, at the time eligibility is determined.

The CPD board, with the advisement of Dr. Frank Newport, editor-in-chief at Gallup, determines which five polling organizations will be used. The CPD board selects polling organizations based on its assessment of poll methodology quality, polling frequency, size of sample population, and the reputation of the polling organization.

So far, the CPD has not reported when it will reveal the five selected polling organizations for 2016, but will likely do so before Labor Day. The CPD has announced it will formally extend invitations to participate in the first debate after Labor Day and before September 26th, when the first debate will be held at Wright State University.

Although the CPD has not yet released the selected polling organizations it will recognize in 2016, we can look back at those included in 2012 for an idea: ABC/Washington Post, NBC News/Wall Street Journal, CBS/New York Times, Fox News, and Gallup.

The key for Gary Johnson is to convince the major polling organizations to include him in their polls—and to continue to do so . He may be in luck. The 2012 organizations that are polling the 2016 election have included him in their match-ups against Clinton and Trump, with Johnson garnering: Fox (12 percent), CBS (11 percent), NBC/Wall Street Journal (10 percent), ABC/Washington Post (7 percent), an average of roughly 10 percent. Other highly regarded pollsters such as CNN/ORC (9 percent) Quinnipiac (5 percent) and Monmouth (9 percent) have also asked about Johnson this cycle, although they were not included in CPD's 2012 recognized polls.

He's not there yet. But Johnson absolutely has a chance of getting to 15 percent in the polls. Let me explain why.

First, NBC/WSJ polls are finding voters in 2016 are more open to expressing interest in a third-party candidate compared to 2012 and 2008. In 2016, 47 percent of Americans say they would consider voting for a third party candidate up from 38 percent in 2008 and 40 percent in 2012.

Second, a Reason-Rupe poll found that 48 percent of Americans say they'd be willing to vote for a candidate who described him or herself as "conservative on economic issues" and also "liberal on social issues," which is how Gary Johnson self-describes.

And third, in 2012, a Reason-Rupe poll found that in a hypothetical three-way race between Barack Obama, Mitt Romney, and Ron Paul, Paul got 17 percent of the vote. And Ron Paul wasn't even running as a third-party candidate and was far better known. Back in 2012, 73 percent of voters had an opinion of Ron Paul—far more than the 32 percent of voters who've formed an opinion of Johnson.

To be sure, Americans are far more likely to express support for minor party candidates in polls than they are at the ballot box. However, public polling is what matters for getting into the debates. These data indicate there is an appetite for a candidate like Gary Johnson, and wherever his ceiling may exist, it's likely higher than 15 percent.

If Johnson is on the eligibility cusp, then the CPD may exercise subjectivity in determining whether or not he is allowed to participate in the debates. For instance, it is unclear how the CPD defines "support…of the national electorate" since in practice pollsters have different methods of determining who likely voters are and thus what is the national electorate.

Also, it's unclear if the CPD would measure Johnson's level of support as a function of a three-way race with Trump and Clinton or a four-way race that also includes Green Party candidate Dr. Jill Stein. The RealClearPolitics average currently finds Johnson with 8.1 percent in a three-way race and with 7.8 percent in a four-way race. One can imagine a scenario where Johnson achieves the 15 percent average threshold in three-way races, but not in 4-way races. If so, it would be up to the CPD's discretion to determine if Johnson meets the eligibility requirements.

What's the incentive of the CPD? It important to point out that the CPD has made great efforts to put forth transparent, concrete and predetermined eligibility criteria well in advance of the debates. The CPD came under scrutiny in the 1990s when it used a broader set of more subjective criteria when deciding whether to invite third-party candidate Ross Perot to the debates. Since then, its board has made efforts to articulate clear eligibility requirements in advance that apply to all candidates.

Nevertheless, one must keep in mind the fact that the CPD was established by the two major parties and initially co-chaired by both the Republican and Democratic parties' national chairman. At the time it was established in 1987 the New York Times reported that both the Democratic and Republican co-chairs individually felt third-party candidates should not be included and thus the CPD would "not likely … look with favor on third-party candidates" and likely exclude them from debates. Because why wouldn't they?

Those who have a vested interest in one of the two major parties have little incentive to allow further competition on the main debate stage. Thus, in the event that a third-party candidate like Johnson finds himself on the edge and subjective judgment must be exercised, Johnson may find himself excluded. However, the intense dislike of Trump, particularly among partisan elites, may lend itself to favor Johnson ultimately getting into the debates this year.

Sam Henrick contributed to this research.