The primary congressional panel tasked with scrutinizing tax proposals has found that a three-year cut in the corporate tax rate would lead to a $489.7 billion drop in revenue over 10 years, creating more head winds for the ambitious tax-cut plan that the Trump administration is working to craft.

The finding, presented to House Speaker Paul D. Ryan (R-Wis.) in a letter from the Joint Committee on Taxation, found that cutting the corporate tax rate from 35 percent to 20 percent in 2018, 2019  and 2020 would lead to a decline in tax revenue collected those years by roughly one-third. And tax receipts would continue falling — albeit only slightly — in subsequent years, in part because companies would rush to repatriate money they are holding overseas during the tax holiday. The result would be reduced taxes on those foreign profits in future years.

While Ryan and other House Republicans are advocating a reduction in the corporate tax rate to 20 percent, President Trump wants the corporate rate lowered to 15 percent, which would lead to an even bigger drop in revenue, although the committee did not provide an estimate for such a change. The White House is expected to reveal more details of its tax plan Wednesday.

“We note that we project a nonnegligible revenue loss in the tax years immediately following the budget window notwithstanding the temporary nature of the tax reduction,” says the letter, which was signed by Thomas Barthold, the Joint Committee’s chief of staff.

The letter illustrates how even a temporary cut in the corporate tax rate could create major revenue challenges. It also shows how difficult it would be for Republicans to pass long-term changes to the tax code. Under Senate rules, a permanent tax cut that is expected to expand the deficit needs 60 votes to pass — a major obstacle for Republicans, who control only 52 seats in the chamber. Without 60 votes, Republicans would be able to pass only temporary tax cuts, and the three-year cut referred to in the Joint Committee’s letter probably would not qualify, because of the reductions in taxes in later years.

The White House has said that its proposed tax cuts would not lead to an increase in the deficit, because they would lead to a boom in economic growth, creating new tax revenue. But when congressional budget scorekeepers evaluate the bill, they won't use that growth assumption to determine whether the legislation will expand the deficit.

Trump administration officials have also said lowering the corporate tax rate will create new incentives for companies to bring jobs back to the United States, and they have said it will also lead companies to bring trillions of dollars being held overseas back to their U.S. headquarters.

The House Republican plan, which seeks a 20 percent tax rate, includes changes to the corporate tax structure that would raise taxes on imports. This change, frequently called a “border adjustment tax,” would bring in roughly $1 trillion in new revenue over one year, according to estimates. The committee's letter did not factor in the potential new revenue that could come from a border adjustment tax.

The Trump administration has not said what types of new tax changes it might seek to offset the big drop in revenue from rate cuts.

The conservative-leaning Tax Foundation, a think tank, has estimated that it would take substantial economic growth to offset Trump's proposed 15 percent corporate tax rate, something it estimated was not achievable without other changes. For example, it projected that the tax cuts would have to lead to an increase in annual gross domestic product by 0.9 percent per year  to create enough revenue to offset the cuts. Rather, it estimates that the cut would lead to an increase in annual growth of 0.4 percent.