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You know that moment on an aircraft where there are three choices of main meal, and you've chosen the chicken and tomato pasta bake.

But by the time the cabin services person gets to your seat, you are told the bake has gone and all you can have is the tasteless salmon and dill or the irradiated, overcooked beef.

Yes, you're disappointed. But you are also immediately in a new mindset. The choice is no longer about what do I want the most. It's what do I mind the least.

The Republican Party establishment finds itself in that position now on the eve of the Iowa caucus.

The palatable, easy-to-digest candidates - a Marco Rubio, a Jeb Bush, even a John Kasich or Chris Christie - are not on the menu.

There is only so long that you can look at the polls and say one of them will break through to challenge the two insurgents, Donald Trump and Ted Cruz.

As things stand, rather than the "moderates" getting together and trying to work out which of them has the best chance to face down Trump or Cruz, they seem to have formed themselves into a circular firing squad and are busy spraying each other with gunfire.

So it's hard to overstate the significance of what has been unfolding over the past few days. It is the grudging acceptance by significant parts of the Republican establishment that not only is Mr Trump the least worst option - he is virtually unstoppable in the race to be their candidate.

The conclusion they've reached is they can live with Mr Trump but they can't with Mr Cruz. Mr Trump will cut deals and compromise; Mr Cruz won't. Mr Trump is biddable; Mr Cruz is not.

Image copyright Getty Images Image caption Bob Dole said Ted Cruz (pictured) would be "cataclysmic" as the candidate

Let me say that again. Unless there is a seismic shift in polling, Donald Trump stands to be nominated as the Republican candidate for the 2016 general election. Potentially the first ever president who has never held elected office or been in the military.

But let me go back to the assertion about the Republican establishment starting to cosy up to Trump. What's the evidence to support that?

As wise old owls, they don't come much wiser than Senator Bob Dole. The 92-year-old, decorated World War Two veteran and former presidential candidate has been there, seen it and done it all. And this week he said that Mr Cruz, a senator from Texas, would be "cataclysmic" as the candidate.

"If he's the nominee, we're going to have wholesale losses in Congress and state offices and governors and legislatures," said Mr Dole, who amassed 35 years' service in the House and Senate.

Meanwhile one of the senators who today epitomises the "establishment" is Orrin Hatch. He says he's "coming round" to Mr Trump. It turns out that Senate Majority Leader Mitch McConnell, from Kentucky, has also had conversations with Mr Trump.

Also, highly unusually, the serving governor of Iowa, Terry Branstad, weighed in. When asked directly whether he wanted to see Ted Cruz defeated, he didn't equivocate . "Yes," he said.

At one of the first Republican debates, Cruz tried to make light of how people saw him: "If you want someone to grab a beer with, I may not be that guy," he quipped. At the time it seemed like self-effacing modesty, but - wow - do the Republican high command loathe him. The ABC strategy seems to be Anyone But Cruz.

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And so, faced with no chicken, tomato and pasta bake, they are swinging decisively against the slippery fish, and going for the leathery, old spray-tanned beef, with a heavy heart.

Because these are the very grandees who commissioned the exhaustive research after the 2012 defeat about how the GOP needed to connect with the ethnic minorities, women and disadvantaged. All groups that - it could be argued - that Mr Trump has been alienating with his rhetoric.

But what he's tapped into is something bigger, which is the profound and visceral anger felt by so many towards that amorphous thing in Washington, "the establishment".

It was brilliantly put by Edward Luce when writing in the FT about Mr Trump's appearance in Iowa earlier this week with Sarah Palin, someone cut from similar cloth: "The more tongue-tied Mrs Palin seemed, the more intently her supporters backed her. The more the media mocked her, the more her fan base exulted. Mr Trump has elevated that approach into an art form. In an age when knowledge is a mark of elitism, ignorance is power. It is also great marketing."

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So can he be stopped? Well, if after Iowa and New Hampshire - the first two states to vote - the circular firing squad of moderate/establishment candidates got together and agreed that there can only be one of them to take on Mr Trump, then maybe. The shortest word in that last sentence was "if", but it is a HUGE if.

The primary process is long and protracted and, as I said earlier, Iowa and New Hampshire are not the pulse of the nation. A lot can happen, a lot can change. Primary history is littered with political mayflies whose wings flutter brightly at the start but live for a very short time. The polls may also be wrong; and the people who profess their greatest support for Mr Trump might not be the people who bother to go and vote. But, but, but.

The polls currently have Trump around 20% ahead of the nearest establishment candidate, with support in the mid-30s - and nothing that has happened along the way these last six months has left a mark on him.

Furthermore, you would have to guess that if Mr Trump's nearest rival, Mr Cruz, pulls out of the race, most of his support will switch to the real-estate mogul.

All of which can only lead to one conclusion - Mr Trump is now going to be extremely difficult to stop.

This might help explain why Republican grandees are starting to make their first tentative moves towards Mr Trump.

And it helps explain why - as often happens on a flight - you end up saying - I'm going to get something to eat when I land.