The Commission on Presidential Debates has taken control of televised presidential debates over the last twenty years. It is formally a non-partisan organization, but it is in reality a bipartisan organization--its current co-chairs are the former chair of the Republican National Committee and the former press secretary to Democratic president Bill Clinton.
It has long established neutral criteria designed to whittle the candidates who appear in the debate to those who are deemed sufficiently "serious" to merit the debate platform. This caused something of a stir in 1992, with the invitation of independent candidate H. Ross Perot, and the waffling over whether to invite him in 1996.
Recently, the criteria have been more objective than some of the earlier balancing tests--but they are also notoriously challenging for any non-Republican-or-Democrat to achieve. It standards for 2016:
[I]n addition to being Constitutionally eligible, candidates must appear on a sufficient number of state ballots to have a mathematical chance of winning a majority vote in the Electoral College, and have a level of support of at least 15% of the national electorate as determined by five selected national public opinion polling organizations, using the average of those organizations' most recently publicly-reported results at the time of the determination. The polls to be relied upon will be selected based on the quality of the methodology employed, the reputation of the polling organizations and the frequency of the polling conducted. CPD will identify the selected polling organizations well in advance of the time the criteria are applied.
A serious problem is what the "five selected national public opinion poll[s]" may mean. That's because, to my knowledge, the CPD has never disclosed its results in recent years when concluding that the two major party candidates have met the criteria. (Consider the generic boilerplate language before the first debate in 2012 here.)
Enter Gary Johnson, the former Republican governor, briefly a 2012 Republican presidential primary candidate, the 2012 Libertarian presidential nominee, and the 2016 Libertarian presidential nominee. Many have hyped that he could attract significant support given the sheer unpopularity of Donald Trump and Hillary Clinton. But would he attract the 15% level of support needed to participate in the presidential debates?
If I had to guess, I'd say no. But Mr. Johnson is currently polling at 8.5% in four-way polling (along with Green Party presumptive nominee Jill Stein) in the Real Clear Politics average. But some polls are as high as 13%; others as low as 5%. In the event he drifts upward a few points, the sample of "five selected" polls matters significantly.
Additionally, his average is slightly higher in three-way polling matchups, polls that exclude Ms. Stein (in part, I think, because the Green Party is not expected to secure ballot access in all fifty states and the District of Columbia)--8.9% in a recent average. While four-tenths of a point may not seem like much, his polling has been about half a point better on average for some time now in the three-way race over the four-way race. Use the three-way polling, or the four-way polling?
If Mr. Johnson experiences some modest improvement in the polls, then the CPD will have almost entirely subjective discretion to decide whether to include or exclude him--if it were include to include him, sample his best-performing three-way polls; if it were to exclude him, sample his worst-performing four-way polls. It's a problem that may not come to fruition if Mr. Johnson's support hovers where it is right now--and maybe that's what the CPD is hoping for.