Donald Trump is set to close this most turbulent year with his first big win. Today, barring a last-minute hitch, both houses of the US Congress will send the president a tax reform bill that he will sign with full ceremony. He’ll lavish praise on himself and say he’s making good on his promise to make America great again. Or as he put it via Twitter: “Biggest Tax Cuts and Reform EVER passed. Enjoy, and create many beautiful JOBS!”

The bill he’ll sign today is indeed the most substantial overhaul to the US tax system since Ronald Reagan’s tax cuts of 1986. Still, Trump and his fellow Republicans should pause before they knock back too much pre-Christmas champagne. This could be a victory that comes back to haunt them.

Play Video 0:28 US House approves Republican tax legislation – video

For this bill hands the Democrats just the ammunition they need for their campaign to win back control of Congress in next November’s midterm elections. It’s unpopular – opposed by 55% of US voters, and supported by just 33% of them – and with good reason.

For this tax “reform” is, in fact, a massive gift to the rich and super-rich. By 2027, 83% of its benefits will go to the richest 1%. Sure, low-income Americans will see a modest improvement in their finances: the poor will save around $60 a year. But the big bucks go to those who already have the biggest bucks.

The mega corporations stand to gain the most, as their taxes fall from 35% to 21%. Republicans are trying to cast this as help for “America’s families and small businesses”, as if the chief beneficiaries will be the Mom and Pop who run the neighbourhood general store. But the reality is that Republicans are paying back the mighty plutocrats who have been bankrolling them for years. Admire the candour of congressman Chris Collins, who last month said of tax cuts, “My donors are basically saying, ‘Get it done or don’t ever call me again.’”

Senate approves most drastic changes to US tax code in 30 years Read more

The stink coming off this bill is even more direct. The men who wrote, passed and will sign it will benefit from it personally. There are tax breaks for real estate tycoons in there, which will further enrich the likes of Senator Bob Corker and one Donald J Trump.

All Democrats need do next November is set those facts against Trump’s 2016 campaign talk, in which he promised to fight for the little guy even at his own expense. “It’s going to cost me a fortune,” he said of his tax plan back then. But it was all a trick. He took the votes of those left behind, and has used them to make himself and his pals even richer.

And they will be the ones to feel the cost. Because you cannot reduce the amount coming into the public coffers by $1.5tn without cutting the amount coming out. There will now be even less money to spend on those who need it. Witness the health programmes for poor children cut for lack of congressional funding on the very day those same Republican congressmen and senators increased to $22m the amount a wealthy couple can leave to their children without paying a cent in inheritance tax.

Perhaps that moral shame doesn’t bother most Republicans. But the party did once pride itself on its fiscal conservatism. They can surely never make that boast again, not when they have passed a measure that will balloon the deficit by a trillion or more. They want to keep spending, including hundreds of billions on defence, even as they bring in so much less.

The right like to say that cutting taxes actually generates more revenue, because people are incentivised to work harder and earn more. But that is one hypothesis that does not need to be tested yet again. We know from the Reagan years, when the deficit mushroomed, that it’s nonsense. It gives magical thinking a bad name.

Trump will brag and crow, but there is a danger here for him too. Passing this tax cut has been the driving mission of the likes of House speaker Paul Ryan for decades. It’s why they’ve tolerated the daily outrages committed by Trump: they were ready to swallow anything for the sake of having someone at that Oval Office desk who would sign their tax bill. Once he’s done it, his usefulness diminishes. Should the Russia probe gather pace, should Trump’s poll numbers go even deeper underwater, then the passing of these tax cuts will lead some Republicans to conclude that he is no longer indispensable.

So let Trump and the Republican party have their celebration today. They may have reason to regret it tomorrow.

• Jonathan Freedland is a Guardian columnist