A group of Democrats in both chambers, led by Sen. Elizabeth Warren of Massachusetts and Rep. Donald S. Beyer Jr. of Virginia, is leading opposition to a potential regulatory carve-out from a new minimum tax on corporate profits that critics say could generate an unjustified windfall for big companies.

The Treasury Department and IRS are weighing a proposal from mega corporations such as Amazon, Microsoft and Ford Motor Co. that would address what they say is an unwelcome, unintended consequence of interactions among three major tax laws dating back to 2017.

If the regulatory boon is granted, these and other companies could get big refunds for research and development spending prior to 2025, which the Democratic critics say would drain the U.S. Treasury for little reason other than to re

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