This Oct. 2, 1978 file photo shows the cast of...

This Oct. 2, 1978 file photo shows the cast of "Gilligan's Island," from left, Russell Johnson, as the professor; Jim Backus as Thurston Howell III; Natalie Schafer, as Mrs. Howell III; Alan Hale Jr., as the skipper; Bob Denver, as Gilligan; Judith Baldwin replacing original cast member Tina Louise, as Ginger, and Dawn Wells, as Mary Ann, posing during filming of a two-hour reunion show, "The Return from Gilligan's Island," in Los Angeles. Credit: AP / WALLY FONG

Why did establishment Republicans get so upset when Donald Trump said his supporters might riot if he is denied the GOP nomination at the convention? Trump fans might riot if their candidate isn’t denied the nomination. They might go wild if concession stands run out of Mountain Dew and “Bomb the @#$% Out of ISIS” T-shirts. It’s what Trump fans do. There are more Trump fan altercations on TV lately than “Seinfeld” reruns.

But establishment Republicans are panicking, and they’re talking about running a “traditional conservative” as a third-party candidate. That’s if their machinations to stop the billionaire real-estate developer fail and Trump is the Republican nominee. Which is exactly what’s going to happen.

So then the plan becomes the third-party Republican thing, which is as appealing to today’s voters as a chicken liver Slurpee. This plan-tantrum, or “plantrum,” springs from the realization that the “establishment wing” of the GOP is just a broom closet. So the prigs in the broom closet, old-line GOPers such as Weekly Standard editor William Kristol and the Club for Growth gang, are acting like stompy-footed children demanding we return the stolen keys to an imaginary kingdom.

Their extra-candidate plan is doomed to fail and guaranteed to turn Hillary Clinton’s win from a substantial victory to a 50-state tsunami of whup.

But I have their candidate.

It has to be an old-school Republican of the country club, Nelson Rockefeller variety. It has to be someone who’s beloved nationally. And it has to be someone whose future political prospects won’t be harmed by unrelenting poll-pummeling and endless jokes on late-night TV.

It has to be Thurston Howell III. Howell was the millionaire with the perfect Locust Valley lockjaw accent who was stranded on “Gilligan’s Island,” a sitcom that ran on TV from 1964 to 1967.

He was everything Mitt Romney (who seems to want one more run) and the other old-school GOPers could ask for. Howell was practically who Romney was as Massachusetts governor, before a sniff of the presidency led him to play a gun-toting, abortion-hating Romneycare-refuting candidate on TV.

But Howell hasn’t been thumped twice already.

Besides Romney, names circulating for this Don Quixote debacle include two-time presidential race sacrificial offering and former Texas Gov. Rick Perry and former Oklahoma Sen. Tom Coburn, who could not beat Flavor Flav in a presidential election. Republicans will have trouble finding a viable nonfictional option. The party ran its top 17 candidates in this race, a process that mostly left them for dead. And House Speaker Ryan is staying away so hard that his new slogan is “My Name’s Paul and This Mess is Between Y’all.”

The “Gilligan’s Island” millionaire played by Jim Backus would be perfect. He knew hard times: During the Depression he went from being a billionaire to just a millionaire. He didn’t vote to authorize the war in Iraq, or oppose it.

And he had the perfect wife for the rabid Michelle Obama haters who abound in the GOP. Her name was Lovey, and she was kind and aloof and never bugged Gilligan and Mary Ann about eating healthy.

Howell really was what so many Republicans were like back in the day. Pro-business, in favor of low taxes, comfortably certain he was a bit better than most, but not consumed with hate or fear or overly concerned with how others lived.

The GOP has just one problem with Howell 2016 — the possibility that, like so many old-school northeastern Republicans, he’s now a Democrat.

Lane Filler is a member of Newsday’s editorial board.