While the clash between Elon Musk and President Trump captivated Washington on Thursday, another drama was playing out behind closed doors over a bill to regulate the $250 billion market for stablecoins, which could transform America’s relationship with the dollar, upend the credit card industry, and benefit both Musk and Trump.

The bill, the GENIUS Act, is poised to pass the Senate within days. But a prominent Republican, Senator Josh Hawley of Missouri, said that he will vote against the bill in its current form, warning that it would hand too much control of America’s financial system to tech giants.

“It’s a huge giveaway to Big Tech,” Hawley said in an interview. Mr. Hawley, who previously voted against the bill for procedural purposes, is concerned that the legislation would allow tech giants to create digital currencies that compete with the dollar.

And he fears that such companies would then be motivated to collect even more data on users’ finances. “It allows these tech companies to issue stablecoins without any kind of controls,” he said. “I don’t see why we would do that.”

Similar worries scuttled an effort by Meta to get into stablecoins. In 2019, Jay Powell of the Fed, among others, raised “serious concerns” about Meta’s cryptocurrency initiative, called Libra and then Diem. It abandoned the project in 2022.

The GENIUS Act has exposed divisions in both parties. Democrats like Senator Elizabeth Warren of Massachusetts oppose the bill, warning it would make it easier for Trump, whose family announced its own USD1 stablecoin in March, to engage in corrupt practices.