This is the second in a four-part series on the televised presidential debates. [Part 1]

When Donald Trump and Hillary Clinton square off next month for their first debate, it's unlikely a third candidate will join them. That's by design, not because voters don't want another option; nearly half say they're open to voting for a third party.

This comes as the Democratic and Republican parties are experiencing a historic lack of support and have chosen, in Trump and Clinton, the most disliked major party presidential nominees in modern times.

But even as the two parties' legitimacy wanes, they maintain control over the televised presidential debates.

"Khyber Pass"

Ralph Nader has called the presidential debates "the Khyber Pass to the electorate" because they offer the only opportunity for third party candidates to reach upwards of 50 million Americans at once.

As the Green Party nominee for president in 2000, Nader routinely addressed huge audiences across the country. Yet "in one debate I would have reached more people, by 50-fold, than I reached by filling all the major arenas," said Nader.

This political "Khyber Pass" used to be controlled by the nonpartisan League of Women Voters, which gave serious consideration to third party candidates. But this angered Democrats and Republicans, who then teamed up to wrest control of the debates away from the League.

The two parties accomplished this by creating the official sounding Commission on Presidential Debates in 1987. Within a year, this private nonprofit corporation had pushed the League aside and taken control of the debates.

"What the Hell?"

Third parties haven't fared well since CPD took over.

When George W. Bush debated Al Gore at the University of Massachusetts in 2000, the CPD had Nader physically barred and threatened with arrest for attempting to watch the debate via live stream in a separate auditorium.

"My immediate thought was: What the hell?" Nader recounted in Crashing the Party.

In the United States of America, I have a ticket to a public function at a public university, and without any cause or disruption, the authorities are throwing me out... See you in court, man.

In Nader's suit, it came out that CPD provided security with a "facebook" containing pictures of third party candidates and their running mates, who were not to be allowed in.

CPD was back at it in 2012. When Barack Obama debated Mitt Romney at Hofstra University, the Green Party's presidential and vice presidential candidates, Jill Stein and Cheri Honkala, were arrested for attempting to enter the debate grounds.

"It shouldn't just be whether or not you have billions of dollars that determine whether or not the American people can hear about your platform," Honkala said shortly before she and Stein were arrested and shackled to chairs for eight hours.

Ross Perot Once, Not Twice

Texas billionaire Ross Perot is the only third party candidate CPD has allowed into the debates, in 1992.

CPD actually fought Perot's inclusion, but was overruled by the major party candidates, George H. W. Bush and Bill Clinton. Bush (mistakenly) and Clinton (correctly) felt Perot would hurt the other, so they told CPD to include him (at the time Perot was polling around seven percent).

The 1992 debates turned out to be some of the most-watched ever, and Perot ended up finishing with 19 percent, the strongest third-party finish since Theodore Roosevelt in 1912.

Four years later, Perot once again stood at seven percent and wanted in on the debates. But this time the major party candidates -- now Clinton and Bob Dole -- wanted Perot out, and CPD excluded him.

After the 1996 election, Clinton advisor (now ABC news anchor) George Stephanopoulos discussed the backroom wheeling and dealing.

[The Dole campaign] didn't have leverage going into negotiations. They were behind. They needed to make sure Perot wasn't in. As long as we would agree to Perot not being in it, we could get everything else we wanted.

With a big lead on Dole, Clinton wanted fewer debates with fewer viewers. And that's what he got. Instead of three presidential debates, there were two, scheduled opposite playoff baseball games. "We wanted the debates to be a non-event," said Stephanopoulos.

15 Percent

As CPD has come under greater scrutiny in recent years, it has increasingly relied on pre-established criteria to determine debate eligibility.

Since 2000, CPD has required candidates appear on enough state ballots to win (which isn't easy), and register at least 15 percent in five national polls (which regularly don't include them).

No third party candidate has made it into the debates since CPD erected its 15 percent barrier.

Despite its exclusionary history, CPD claims it's open to having a third candidate participate in the upcoming debates. "It would be great," said CPD co-chair Frank Fahrenkopft.

Regardless, in this 'Year of the Outsider,' CPD's barriers are likely to be tested as never before.

Next up: Can Jill Stein and Gary Johnson Crash the Debates?