Republicans in Washington are about to embark on arguably their most important – and complex – single agenda item of Donald Trump’s first term: tax reform. What could possibly go wrong?

The fever of anticipation across the land is matched only by the fever of nervousness on Capitol Hill. If Republicans screw this up like they did replacing Obamacare they face being routed in mid-term elections next year. Leading the charge against them will be Steve Bannon, former West Wing strategist, who is already waging a private war on Senate Majority Leader, Mitch McConnell, and many of his colleagues up for re-election whom he deems insufficiently conservative.

The economy would be disappointed too. Much of the steam that has kept the stock market powering ahead in spite of the political turmoil in Washington and growing global threats has been pumped by the mere expectation of the tax overhaul about to get under way. Confidence could drain like water from a bath should it become clear it might not happen after all.

Look at what’s being dangled. First, a far simpler tax code. Trump’s daughter, Ivanka, said on Monday even she doesn’t understand the code as it is now with its multiple earnings brackets and array of opportunities to lower your tax bill with deductions and other ruses. Trump wants filing taxes to be easier than rustling up a macaroni cheese. Or maybe a Turducken.

Then there is the promise that American families – the great middle class – and American businesses will be looking at much lower tax bills next year. Who wouldn’t love that? Trump wants to lower the corporate tax rate from 35 per cent today to 20 per cent. The White House claims that that alone would translate into $4,000 a year more for the average household.

How did they arrive at that figure? Well, who knows? That is the thing about rewriting the tax code, it’s devilishly complicated and often the consequences are hard to predict. That’s why no one in Washington has attempted it for over 30 years. The minute you embark on the process, huge economic and ideological arguments flare up. Like the one about trickle-down: slash taxes on the rich and everyone will benefit. Myth or real? (Almost all the evidence says the former.)

If the Republicans don’t get it done, they will be crucified next year. That would mean losing control of the Senate and possibly even of the House of Representatives where they have a comfortable majority now. If they do get it done, they risk being crucified also. Because how do they deliver on all these promises without exploding the federal deficit? They are talking about cuts in taxes of about $1.5 trillion, but the package could cost Uncle Sam far more than that.

Remember, it was runaway federal spending and lack of budget discipline during the George W Bush years that got so many grassroots Republicans riled up at a decade ago, sparking the populist appeal of folk like Sarah Palin and the rise of the Tea Party. Who's to say what kind of insurrection Bannon could provoke if in their desire to slash tax rates – and therefore tax revenue – McConnell and his crew end up sending the deficit rocketing like never before?

For now, though, it’s not getting anything done at all that petrifies the party the most. And several things are already threatening the process. Included among them is Trump. Add to that precisely a growing realisation that you can’t depress tax revenue to the extent being contemplated without risking a massive hike in the deficit. And there’s Trump.

Did I mention Trump twice? Unity in the ranks will be essential, including between the Republicans on the Hill and the Republican in the Oval Office. For anything to pass, they will need every Senate Republican on side. The Democrats won’t help. (Remember Obamacare, again.) Yet, as we go into the process, the President is already engaged in feverish feuds with several senior figures of his party, among them Bob Corker, chairman of the Foreign Relations Committee. The same Bob Corker, who “couldn’t get elected dog catcher”. That’s what Trump said about the Senator from Tennessee on Tuesday after he had gone on TV to suggest that the president keep his nose out of tax reform and “leave it to the professionals”.

Corker was infuriated that Trump had just barged into the process via Twitter ruling out an end to existing tax deferments for Americans investing early in so-called 401(K) retirement accounts. It had been singled out as one way to help offset the huge losses of tax revenues likely elsewhere. He killed it dead with one tweet. “There will be NO change to your 401(k),” he said.

This is the reality Republicans are now facing. Trump wants a giant tax give-away ready in time for Christmas without any associated political pain. He will push back any time they try to assert at least a modicum of budget discipline by removing existing goodies in the tax code that are popular with his base, like the 401 (K) tax deferment.