In some ways, it was typical Donald Trump: He belittled the other candidates (“are these people stiffs or what?”), blasted US foreign policy leaders (“a bunch of weak sisters”), and then bragged that he “built a lot of great wealth.”

But then, surprisingly, he made a valid point: “It’s disgraceful” that third-party candidates are systematically excluded from the nationally televised presidential debates.

“I am not surprised that the two-party political establishment wants to keep the American people from having a third choice,” Trump said. “It’s amazing that they can get away with it.”

That was January 2000. The celebrity real estate magnate was flirting with a presidential run on the Reform Party ticket. Sixteen years later he’s the presumptive Republican nominee, and the mere hint of “a spoiler indie candidate” drives him into a spluttering rage on Twitter. Sad!

When Trump was a political outsider, he wanted the debate stage opened up to alternative viewpoints; now that he’s a member of the club, he wants it kept more exclusive than the Mar-a-Lago.

The leading third-party alternative in the 2016 race is Gary Johnson, the former New Mexico governor who has clinched the Libertarian Party nomination. Johnson has reached as high as 12 percent in the few polls that have bothered to mention his name, and he’s likely to be on the ballot in all 50 states.

The Donald of 2000, with his expert’s eye for a fraudulent scheme, was absolutely right that the system is rigged to exclude third-party alternatives.

The fix had been in since at least 1988, when the major-party front group known as the Commission on Presidential Debates engineered a hostile takeover of the debates, shoving aside the independent group that had hosted them from 1976-84, and ensuring that the two-party “duopoly” would control the process in order to preserve politics as usual.

This year the CPD will do everything in its power to make sure the national debate audience never gets to hear Johnson.

As Trump said, “it’s amazing that they can get away with it.” Understanding how they managed the feat requires a look back at the corrupt bargain that gave rise to the CPD and enabled it to hijack the presidential debates.

From 1976 through 1984, an independent group, the League of Women Voters, hosted the debates, and repeatedly rebuffed major-party demands for safe, stage-managed affairs.

In 1980, when President Jimmy Carter refused to appear with independent candidate John Anderson, the league threatened to hold the debate with Anderson, Ronald Reagan, and an empty chair. (They eventually went forward sans the chair — and Carter.) During the 1984 race, after the Reagan and Mondale teams rejected 68 proposed debate panelists, the league backed them down with a press conference calling out the campaigns for “totally abusing the process.”

That sort of behavior struck major-party moguls as entirely too “pushy.” They wanted a more compliant sponsor, and if you want something done right, sometimes you’ve got to do it yourself. Thus, in 1984 the chairs of the Republican and Democratic national committees hatched a plan to sideline the league and take over the debates: “The two major political parties should do everything in their power to strengthen their own position,” explained then-RNC head Frank Fahrenkopf. Three years later, the parties announced the formation of the Commission on Presidential Debates, co-chaired by … the heads of the RNC and DNC.

In 1988, the league refused to go along with a restrictive “Memorandum of Understanding” that set the terms of the Bush-Dukakis debates, warning that it “would perpetrate a fraud on the American voter.” The CPD stepped in as official sponsor, and the takeover was complete.

Negotiated between the campaigns every four years and rubber-stamped by the CPD, the candidates’ Memoranda of Understanding read like Hollywood stars’ contract riders.

The 2012 Obama-Romney MOU is typical; at 21 pages, it covers minutia like the specific placement of the podiums: “equally canted toward the center of the stage” at an angle to be approved by the campaigns.

But the real problem is what the MOUs restrict. “In general, direct candidate-to-candidate questioning has been banned,” reports an Annenberg white paper on debate reform, and there are to be no “challenges for additional debates.” The moderators are prohibited from asking the candidates for “‘a show of hands’ or other similar calls for response,” and in town hall debates, follow-up questions are prohibited.

Even the camera crew is under tight restrictions: “No TV cut-aways to any candidate who is not responding to a question.” Perhaps the Trump campaign can add a proviso ensuring that the cameras never linger on the candidates’ hands.

The CPD would, no doubt, be willing to oblige. As Scott Reed, Bob Dole’s campaign manager in 1996, explained: “The commission does what you tell them to do,” including barring the debate forum door to any candidate who might spoil the party for the red and blue teams.

In the 1992 cycle, Texas billionaire Ross Perot had been included in all three debates at the insistence of the George H.W. Bush campaign, which wrongly expected he’d tip the race to Bush. Perot shot up from 7 percent in pre-debate polls to nearly 19 percent of the popular vote on Election Day.

But in 1996, both the Clinton and Dole campaigns wanted Perot kept off the stage, and the CPD complied, even though three-quarters of eligible voters wanted him included. The parties got their way, and managed to duck the blame for it as well: “We were able to hide behind the commission,” said Reed.

To make future exclusions look less arbitrary, in 2000 the CPD adopted a numerical standard: Eligible candidates would need to show at least 15 percent support in independent national polls in the runup to the debates. To have such “a high criteria for a party that’s a legitimate party” that will be on the ballot “in all 50 states [is] very unfair,” Trump complained at the time. Indeed, that requirement kept the Reform Party’s eventual nominee, Pat Buchanan, and the Green Party’s Ralph Nader off the stage that year, and would have barred Anderson in 1980, Perot in ’92, and nearly every third-party candidate in American history.

The 15 percent rule will keep Gary Johnson out too, unless he’s able to better his current standing in the polls. But as Perot showed in 1992, sometimes admission to the debates is a prerequisite for cracking that barrier. Another Catch-22 for Johnson is that, thus far, most national polling organizations aren’t asking about him — and the CPD picks the pollsters that count.

So what’s Trump so worried about? The Donald likes to posture as a fearless outsider — he even wrote a book called Time to Get Tough (chapter one: “Get Tough”). Lately, though, he gets jumpy whenever the Libertarian candidate comes up in an interview: “I don’t want to mention the name; we want to give them as little publicity as possible,” Trump said on Fox News the other day.

Luckily for the Donald, he’s an establishment insider now, and the Commission on Presidential Debates is on his side, ready to protect insiders from competition. Membership has its privileges.

Gene Healy is a vice president at the Cato Institute and author of The Cult of the Presidency.