House Speaker Paul Ryan, R-Wis., thinks he has a simple way to raise roughly $1 trillion to pay for his long-sought overhaul of the tax code, but many businesses — and even some Republican lawmakers — are wary of his plan to change how the U.S. treats imports and exports.

Ryan first floated “border adjustability” in the “better way” plan he unveiled last summer outlining how the GOP would do everything from repealing and replacing the Affordable Care Act to roll back federal regulations to revamping the tax code.