The moment is almost upon us: seated at a grand desk in the White House a beaming Donald Trump will shortly wield his pen to sign the first major rewriting of the American tax code in decades into law. Actually, according to tradition, he will use several pens, which he will then distribute to his Republican friends from Capitol Hill as souvenirs of their moment of victory.

The smiles in the room will have more wattage than the Christmas tree lights at Rockefeller Center. And behind them will reside the answer to one of the more urgent questions of the year: why has the party of Reagan and Lincoln been so loathe to criticize, let alone openly challenge, a leader who has done so much to sully its reputation and undermine its moral standing?

The reason is suddenly obvious. It’s not because Paul Ryan, the speaker of the House, and Mitch McConnell, his counterpart in the Senate, are just craven or weak. They maintained a front of virtual indifference to Trump’s assorted sins - you know what they are by now - not because of mere feebleness. It was because they never lost faith that this day would come.

Look, even those few in the party who have made stands of one sort or another against him fell into line to give the tax bill life. John McCain won’t actually vote because of ill health. But he declared his support for it. Jeff Flake and Bob Corker, both of whom are retiring from the Senate, made brief shows of opposition before promising to vote yea. You thought moderates Susan Collins and Lisa Murkowski of Maine and Alaska might resist? You thought wrong.

It’s perfectly possible the Republicans are walking into a trap. Because they failed to bring even one Democrat on board, they alone will own this. And there is plenty of reason to suspect that it will not turn out to be the giant Yuletide gift to the middle class that Trump says it is.

In fact, the backlash has already begun. A CBS News poll this week showed how unpopular it has become even before its completion. Only one in five Americans believe Trump’s pitch that it will lower their annual tax bills. And three quarters of them have become convinced that by far the biggest beneficiaries of it will be mega-corporations and those who are already super-rich.

That this is the perception is not surprising. Since it’s accurate. A new report by the non-partisan Tax Policy Center said that while it would lower taxes for 95 percent of Americans next year, on average the cuts for the highest earners will far exceed those of everyone else. Moreover, those cuts that may help the middle class will expire after just a few years. In ten years time, 53 per cent of Americans will pay more in taxes. The cuts in corporate taxes have no expiration date.

The way it has been written, this law gives an open door to Democrats who, in fighting to regain control of Congress next November, will focus like a laser on the notion that Republicans care only about the very rich and that Trump’s promise to help the forgotten is no more than a fraud.

Yet the Republicans see nothing not to like. Trump has trampled on a multitude of things they are meant to hold dear - free trade, engagement with the allies, general rectitude and on and on - but on this one issue he has played with the orchestra. Harnessed together, Trump, McConnell and Ryan are delivering on a single, familiar orthodoxy: that you help the middle class by helping the rich first. Giving to the haves will eventually help the have-nots.

Indeed, no strand of the Republican DNA is more strong or enduring. At one point, this supply-side approach to boosting prosperity was called Reaganomics (even though Reagan was eventually forced to raise taxes after slashing them because he ran out of money). But it dates back even further to Presidents Harding and Coolidge in the 1920s. No amount of evidence that trickle-down does not in fact work as advertised has cooled the party’s ardour for it.

Adding to the general air of jubilation on the GOP benches is the dirty truth that skewing the tax equation this way will be of immediate and direct benefit to the very rich folk who will be donating to their campaigns next year. It’s been a pretty miserable year for cash inflows to the party coffers, but watch the taps spring open the moment the ink from Trump’s pen is dry. For those members of Congress thinking about retirement in favor of some high-paid job in the private sector, the opportunity to ensure they'll pay less in taxes surely had its appeal too.

That the law also rescinds the mandate at the heart of Obamacare requiring most Americans to buy health insurance is just the icing on the cake. The Republicans failed horribly in their attempts to repeal the healthcare law earlier this year. This doesn’t quite do that, but may very well have much the same effect. Without the mandate obliging all Americans to buy policies, premiums will soar making it all but certain that Obamacare will self-destruct all on its own.