Still, she added, the 15 percent “floor” could be too high for some countries to accept and too low to win approval from some members of Congress in the United States.

Rohit Kumar, leader of PwC’s Washington National Tax Services office, said that the reaction from Ireland and other countries to the proposal will be crucial because a tax agreement reached through the negotiations would be far from ironclad.

“Do countries actually change national law and enact it? Or is it just a political agreement where everyone is says, ‘That’s nice, but we’re not doing it?’” Mr. Kumar, a former top aid to Senator Mitch McConnell, the Senate minority leader, said. “As U.S. lawmakers are examining these proposals, that is the several trillion dollar question.”

Treasury officials said that they never insisted on the 21 percent rate, but that they believed that other countries were receptive to the idea of adopting a rate higher than 15 percent depending on the fate of the changes to the American tax system that are under consideration.

Ms. Yellen has warned that a global “race to the bottom” has been eating away at government revenues, and she has adopted a more collaborative approach to the negotiations than the Trump administration employed.

She is expected to continue talks about global tax reform with her international counterparts at the Group of 7 finance ministers meeting next month.

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