One simple way to find the money that Mr. Corker is requesting would be to reduce the corporate tax rate, not to 20 percent, but closer to 25 percent; the difference between the rates is a bit under $500 billion over 10 years, said Scott Greenberg of the independent Tax Foundation. For a similar price, senators could choose to phase in, over a decade, their cut to 20 percent from 35 percent, a move favored by conservative economists like Alex Brill of the American Enterprise Institute.

But Mr. Trump has drawn what the White House calls a red line in negotiations at a rate of 20 percent, putting Republicans in something of a box.

Spurred by Mr. Trump, many conservative groups are insisting on a 20 percent rate, which they say is critical to making the United States more competitive with other developing nations that have relatively low corporate rates. In a letter on Thursday, several groups in the conservative Koch network warned that a rate any higher than that would undermine the economic growth goals of the bill.

“Business tax relief is central to the goals of creating jobs and promoting economic growth,” they wrote, “and we are concerned that any effort to hold the corporate tax rate above 20 percent will substantially limit our ability to achieve those goals and expand opportunities for all Americans.”

The Koch network was instrumental in defeating a proposal from House Republican leaders earlier this year — known as border adjustment — that would have raised additional revenue by taxing imports. Eliminating that provision was a central focus of the network’s semiannual donor retreat in Colorado over the summer; officials there warned that it would drag on economic growth.