Treasury Secretary Steven Mnuchin speaks to the media during the daily briefing at the White House. (Pablo Martinez Monsivais/AP)

The White House’s proposal to overhaul the tax code would lead to the biggest tax cut in American history, Treasury Secretary Steven Mnuchin said Wednesday as part of the Trump administration's bid to build support for one of its top domestic priorities.

Mnuchin said some of the key details of the plan remained in flux, such as whether they would seek temporary changes to the tax code or a long-term change. But he said outlines of the plan, which will be released later on Wednesday, would call for sweeping changes and reductions in rates.

“We want to simplify the personal tax system, lower taxes and create economic growth,” Mnuchin said, speaking at an event hosted by The Hill. “So this is going to be the biggest tax cut and the largest tax reform in the history of our country and we are committed to seeing this through.”

He confirmed reports that the White House would propose lowering the corporate tax rate from 35 percent to 15 percent. He also confirmed that the White House would call for allowing millions of small businesses to also take advantage of this lower rate, though he said certain protections would be put in place to prevent high-earners from gaming the system to lower their personal taxes.

President Trump unveiled his tax plan on April 26, after months of pledging to make drastic changes to the tax code. The Post's Damian Paletta explains why tax reform is so complicated. (Jenny Starrs/The Washington Post)

Some Democrats were immediately skeptical. Senate Minority Leader Charles Schumer (D., N.Y.), said members of his party would scrutinize the details, but he predicted the package could amount to major tax breaks for the wealthiest Americans and for businesses like those formerly run by President Trump.

"That's not tax reform," Schumer said on the Senate floor. "That's just a tax giveaway to the very, very wealthy that will explode the deficit."

Mnuchin also addressed one of the most controversial elements of the current tax debate, a proposal from House Republicans to change the way taxes are collected from businesses that would essentially raise new money with higher taxes on imports. This proposal, known as the “border adjustment tax,” has split Republicans.

Trump's White House won’t support this specific House GOP proposal, but Mnuchin said they would work with Republicans to revise the idea in a way that it could eventually become part of the overhaul.

“We don’t think it works in its current form,” but the White House will work on “revisions that they would consider,” Mnuchin said.

He didn’t address what the deficit impact would be of the tax cuts. Many budget experts believe tax cuts of the size Trump will propose would lead the government to lose trillions of dollars in revenue over 10 years, ballooning the government debt. Mnuchin did not specify any way of raising new tax revenue to offset the big cuts in tax rates.

“What I don’t want to see is that this tax reform is going to be paid for by magic,” said Maya Macguineas, president of the Committee for a Responsible Federal Budget, a group that advocates for deficit reduction.

But Mnuchin said the goal of the tax plan would be to create so much economic growth that it would create new revenue on its own.

“This is the center in the core of his economic plan,” Mnuchin said of Trump.

Mnuchin said the White House prefers for the tax changes to be made permanently, but said there were other options depending on what was palatable for members of Congress.

“If we have [the tax cuts] for 10 years, that’s better than nothing,” Mnuchin said. “But we’d like to have permanency here.”

Mnuchin said Trump has been closely involved in the design of the plan, meeting with advisers in the Oval Office about the package on Tuesday. Trump called Mnuchin at 7:30 Wednesday morning to discuss the specifics again.

Changes to the tax code must be approved by Congress, and a major overhaul of the code hasn’t been done in more than 30 years. Top White House officials have little experience pushing a legislative package like this through Congress, but they are determined to see it through, Mnuchin said.

“We’re committed to working, getting this thing done, getting everyone in the room, and getting this thing passed,” Mnuchin said.