The US doesn't need to change any laws to meet this agreement. We already tax our corporations more than 15%. What the US wants is for other countries to tax that much, to discourage our own multinationals from booking revenue outside the US to avoid US tax. The EU wants companies to book revenue where they make it, which they can do all on their own. They don't need the US for that.
And what makes you think the GOP wouldn't support this? It would give them cover to lower the tax rate to 15% from 21% to "be in line with the rest of the G7". Also, if our multinationals can't avoid tax anymore, there is a good chance they would just book their revenue here in the US, leading to more revenue for the US and less for Europe.