“We are going to have the largest tax cut ever,” Donald Trump claimed earlier this month – the latest in a series of promises of tax reform from the US president. But as the plan finally emerged on Thursday it may be the president himself who is the biggest threat to the bill getting passed.

The long-awaited $1.51tn plan to cut taxes for corporations, reduce them for some middle-class families and streamline the tax code looked more costly, more vulnerable and smaller than originally planned.

Trump made good in these proposals on his promise to lower the corporate tax rate from 35% to 20% and eliminate the estate tax, and the alternative minimum tax, two taxes that affect only the very rich.

So-called pass-through businesses, businesses that are taxed at the tax rate of the individual business owner, will pay a 25% tax rate, lower than the 39.6% paid by top-rate taxpayers.

A similar plan to this created a vast hole in Kansas’s budget that forced the state to cut spending on schools, roads and other essentials. The Republican plan aims to stop some of the abuses seen in Kansas, where it appears to have been used as a tax dodge by the wealthy. But no matter what they do they are unlikely to change the perception that this is just another handout for the rich. Trump himself controls 500 such businesses and the plan hands him a significant tax break.

Duane Goossen, the former Kansas secretary of administration, said he got a sense of deja vu when he saw the tax plan talking points. “We were promised tax cuts for all, more businesses and growth,” he said. “What we got was tax cuts for the rich and a budget crisis.”

About 49% of all pass-through income goes to the top 1% of households, according to the Economic Policy Institute.

“In the meantime, some middle-class families will end up paying more in taxes because of the loss of popular deductions, while corporations will get a multitrillion-dollar windfall,” said TJ Helmstetter spokesman for lobby group Americans for Tax Fairness. “It’s outrageous and immoral, and the GOP is going to have a hell of a time sneaking this past the American public.”

In another blow, the National Federation of Independent Business, the influential lobby group for small businesses, said it was “unable to support” the bill and called on lawmakers to “make the necessary corrections”.

“This bill leaves too many small businesses behind,” the NFIB president, Juanita Duggan, told the Wall Street Journal. “We are concerned that the pass-through provision does not help most small businesses. Small business is the engine of the economy. We believe that tax reform should provide substantial relief to all small businesses, so they can reinvest their money, grow, and create jobs.”

In order to pay for the plan the Republicans are planning swingeing cuts to Medicare and Medicaid, the public healthcare schemes for older and low-income Americans. The plan will also cut back on tax benefits for millions of US families, especially homeowners in high-cost housing areas – a move that could backfire spectacularly.

And that is where the plan could really run into trouble. Those cuts are already under fire from one powerful – and well-funded – lobby group that should really have Republicans worried: homeowners.

Under the bill, homeowners would only be able to deduct interest payments made on their first $500,000 worth of home loans on new mortgages, down from $1m.

“This plan will hurt millions of hard-working American families and marginalize homeownership. The American people deserve better,” Granger MacDonald, chairman of the National Association of Home Builders (NAHB) said at the weekend.

The NAHB is regarded as one of the most powerful lobby groups in Washington and its fight will be backed by other high-spending lobby groups including the National Association of Realtors, which recently targeted every district of the House ways and means committee’s members with ads asking constituents to remind the lawmakers not to “let tax reform become a tax increase for middle class homeowners”.

Homeowners have already been shown a path to victory. The tax plan had originally aimed to cut the amount of money people can put into tax-deferred 401(k) retirement savings plans. But amid stinging criticism Trump nixed that plan.

“There will be NO change to your 401(k). This has always been a great and popular middle class tax break that works, and it stays!” he tweeted.

But someone has to pay for these tax cuts, and fiscally conservative Republicans are likely to defect if they see the plan doing little but adding to the national debt. After the bill was released, the non-partisan Committee for a Responsible Federal Budget called it “a bill of treats paid for by too many tricks that could harm the economy” and add $12,000 in debt to every US household.

If Trump caves to pressure from the home builders, and other offended groups, just as he did with 401(k)s, the “biggest tax cuts in history” may soon be history themselves.