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

But it's not the first time the administration has promised an imminent plan, and the obstacles to a sweeping tax-code rewrite of the sort Mnuchin described Thursday haven't gotten any smaller. A key Senate committee has yet to see final details of a White House plan, a congressional aide said. And tax-related challenges presented by the 2010 Affordable Care Act remain in place amid Republicans' disagreement on how to dismantle the health-care law they'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 their exports — has stirred controversy among U.S. businesses, created conflict among Republicans and has yet to win President Donald Trump's endorsement.