Treasury Secretary Steven Mnuchin sent U.S. stock prices to adaytime high Thursday when he said the Trump administration willproduce an ambitious plan to overhaul the U.S. tax code “soon.”

But it's not the first time the administration has promised animminent plan, and the obstacles to a sweeping tax-code rewrite ofthe sort Mnuchin described Thursday haven't gotten any smaller. Akey Senate committee has yet to see final details of a White Houseplan, a congressional aide said. And tax-related challengespresented by the 2010 Affordable Care Act remain in place amidRepublicans' disagreement on how to dismantle the health-care lawthey've criticized for years.

On top of that, House Speaker Paul Ryan's proposal to tax U.S.companies' domestic sales and imports — while exempting theirexports — has stirred controversy among U.S. businesses, createdconflict among Republicans and has yet to win President DonaldTrump's endorsement.