At last, the Republican establishment is engaged in a project the entire world can get behind: the campaign to stop Donald Trump. They’ve left it desperately late, but a collective effort is finally under way to prevent the would-be strongman – who boasts of his admiration for Vladimir Putin and insists he will order US soldiers to commit war crimes, torturing prisoners and killing the innocent families of terrorists – from becoming their party’s standard-bearer in November.

The trouble is, nothing seems to work. In fact, it’s worse than that. Trump is coming to resemble a character from a comic-book: not only do bullets bounce off him, they make him stronger. On Thursday, his enemies mounted a double assault. In a move without precedent, the party’s previous nominee – 2012 candidate Mitt Romney – gave a speech that declared this year’s frontrunner unfit to be president. In a more damning indictment than he ever unleashed against Barack Obama, Romney branded Trump a phoney, a fraud, a bankrupt and a misogynist.

Later that night, during yet another televised debate, Trump’s main rivals, Marco Rubio and Ted Cruz, formed a tag team to pile on Trump, slamming him as a liar and a fake. And that was during the evening’s loftier moments. (Let historians note that one CNN headline read: “Donald Trump defends size of his penis” and leave it at that.)

Rule-breaking defiance is central to Trump's appeal – nothing in the conventional armoury will work

Yet few believe any of this will make much difference, still less halt the frontrunner. Trump himself just brushes it off, usually by insulting his accusers back. He does the same when confronted with evidence of his dizzying inconsistency and policy U-turns, often simply talking over his inquisitor, drowning them in bluster.

But there’s a reason these punches don’t land. Romney’s critique was calm, coherent and indisputable. Yet he was utterly the wrong messenger. Trump is running as the outsider, the man who dares take on the political and corporate establishment. If that establishment had a face, it would look like Mitt Romney. Trump rails against the outsourcing of jobs away from the US: it was outsourcing that helped Romney make his fortune. (And helped Trump too: note the “Made in China” labels on Trump’s “Signature” shirt collection.)

The same goes for the new Rubio-Cruz alliance and the wider strategy it represents. Current Republican thinking is that if Trump can be denied an outright majority of delegates to the party’s convention in July then it might be possible to deny him the nomination. To that end, the non-Trump candidates now aim tacitly to help each other win where each is strongest, thereby keeping the lid on Trump’s tally.

Yet surely this plan too is doomed. Not just because it relies on Trump stumbling and his rivals doing well, even though the polls show, say, Rubio struggling in his home state of Florida. The problem is that by ganging up against him, Rubio, Cruz and John Kasich once again confirm Trump as the outsider. They present themselves as interchangeable creatures of the establishment, resorting to political games – the whole rotten system to which Trump pretends to be the answer.

Facebook Twitter Pinterest A Donald Trump supporter at a campaign rally in Cadillac, Michigan. Photograph: Jim Young/Reuters

For the British observer, it stirs a distant memory of the Labour establishment effort to stop Jeremy Corbyn last summer. Every intervention they made – every dire warning issued by a former leader or grandee – only entrenched Corbyn’s status in the eyes of his devotees as the insurgent outsider who had rattled the powers-that-be. The Republican elders find themselves in the same hole now.

This is the problem now looming over established parties around the world. When facing candidates who revel in breaking every political rule – indeed, whose rule-breaking defiance is central to their appeal – nothing in the conventional armoury will work.

The Republicans’ deep-pocketed donors are about to sink millions into anti-Trump TV advertising between now and the next big voting day on 15 March. But they know that Jeb Bush tried that already and it made not a dent. (One of the few upsides of this vicious campaign: the once decisive TV ad seems to have lost its bite.)

Some think the best way to pay back Trump is in his own coin. Rubio has been trying that, hurling schoolyard insults with vigour: the penis size thing started with him, when he mused out loud about Trump’s small hands. But that serves only to diminish the entire contest, sinking it to the gutter level where Trump is both more skilled and experienced. Little Marco, as Trump calls him, is learning afresh that you should never wrestle with a pig – you both get dirty, and the pig likes it.

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There are of course some arrows that might just pierce the Trump armour. For example, in a brilliant monologue, the TV satirist John Oliver pointed out a few of Trump’s most egregious hypocrisies. Not just the outsourcing by this supposed champion of American jobs, but the string of business failures that puncture Trump’s self-invented myth of gaudy, gargantuan success.

Still, the way to defeat Trump will not come through clever tactics or even the sharpest mockery. It will only come by understanding the roots of his appeal. Many of those rallying to his flag now are white, blue-collar Americans angry about jobs lost or wages frozen through globalisation and free trade; angry about faraway wars that seem to make no sense and fail to bring the catharsis of victory; and angry about an elite class apparently able to write the rules for its own benefit, gobbling up the wealth and leaving the taxes to be paid by the little people.

Trump’s Republican opponents can’t offer a remedy for those three wounds: they are too implicated in each of them. Indeed, as former Republican Robert Kagan argued this week, Trump is a Frankenstein’s monster of the party’s own creation. Through the intensity of Republicans’ anti-government rhetoric, their disdain for compromise and the very business of politics, and their indulgence of bigoted attitudes to immigration, they rolled the pitch on which Trump is now playing.

Which means it will probably be left to the Democrats to defeat him in November. Bernie Sanders has long been addressing that same trio of grievances – declining jobs, foreign wars and Wall Street excess – but the task is likelier to fall to Hillary Clinton.

It will be her duty to defeat Donald Trump. And she will have to do it first by exposing him for the nasty, bullying dictator-in-the-making that he is. But she will also have to understand and address the pain that is drawing so many Americans to his ugly, dangerous flame.