DONALD TRUMP has proclaimed his White House to be the most productive since Franklin Delano Roosevelt. But with the symbolic 100-day mark fast approaching, he has yet to pass a single major piece of legislation. Mr Trump is hoping that a “massive” tax plan, the broad outlines of which were unveiled on April 26th, will give him his first legislative victory. But without significant changes, the plan will be temporary. And getting it passed will be difficult.

The centrepiece of Mr Trump’s package is an overhaul of the corporate tax system—something most policymakers agree is badly needed. Even after accounting for a myriad tax breaks, companies in the United States pay more taxes than many of their rich-word counterparts. Lowering the corporate tax rate while broadening the tax base, budget experts agree, would make the tax system more efficient and boost economic growth.

Unfortunately, Mr Trump’s plan focuses on only one side of this equation, calling for a major tax cut without identifying a way to pay for it. Trump wants to cut the corporate tax rate from 35% to 15%, a change the Congressional Budget Office reckons would cost more than $2trn over the next ten years (roughly 5% of all federal revenues). On the campaign trail, Trump also proposed allowing companies to deduct the full cost of capital investments in their first year, which would cost another $600bn over the next decade.

The president has said he will pay for these cuts by eliminating “most corporate tax expenditures”. But government figures suggest that repealing every corporate deduction, exemption and exclusion on the books—other than the few tax breaks he has said he would not touch—would generate roughly $126bn per year, enough to pay for less than half of Mr Trump’s proposed reforms. According to Steven Mnuchin, the treasury secretary, the administration’s tax plan will “pay for itself” through faster growth. This may be another case of truthful hyperbole. Any effects that the proposed tax cuts may have on growth—measured using a technique called “dynamic scoring”—would be modest. Revenue generated from a one-time repatriation tax holiday would be temporary.

Mr Trump’s tax plan is therefore likely to increase the deficit. This presents a significant problem for its passage. Under Senate rules, any bill that increases the deficit beyond the ten-year budget window cannot be passed with just 51 Republican votes through the so-called reconciliation process. If Mr Trump wants a permanent tax cut that isn’t fully paid for, he must win over Senate Democrats. Even for the “biggest tax cut” in history, this seems unlikely.