President Trump's forthcoming tax plan will be the "biggest" tax cut in history, Treasury Secretary Steven Mnuchin said Wednesday at an event hosted by The Hill.

Mnuchin provided several clues as to what will be in the package, which will be unveiled later in the day.

He confirmed that the proposal would cut tax rates for businesses to 15 percent. That rate would apply to both corporations and owner-operated businesses known as "pass-throughs."

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The 15 percent rate was also part of Trump's campaign plan. Mnuchin said that Trump thinks that's "absolutely critical" for driving economic growth.

He also said that the administration wants to simplify the personal tax system and that most Americans should be able to file their taxes on a large postcard.

"The average American should have simple taxes," Mnuchin said, adding that many people won't end up paying any taxes under the administration's plan.

House Republicans have been working on a tax bill based on a plan they released last year. That blueprint includes a controversial provision known as "border adjustment" that would tax imports and exempt exports.

Mnuchin said that the White House doesn't think the border-adjustment provision "works in its current form." He said that the administration will continue to discuss potential revisions to the proposal with lawmakers.

Mnuchin also said that Trump's proposal won't include infrastructure spending.

"This plan is just tax reform," he said.

Trump has floated linking tax reform and infrastructure in the past, but some conservatives have argued that the two issues should be kept separate.

Americans for Tax Reform President Grover Norquist said in a panel after Mnuchin spoke that the decision not to mix taxes and infrastructure was a "big step forward," since infrastructure spending would provide fewer resources for lowering tax rates.

Mnuchin said that he's been meeting with House Ways and Means Committee Chairman Kevin Brady Kevin Patrick BradyBusinesses, states pass on Trump payroll tax deferral Trump order on drug prices faces long road to finish line On The Money: US deficit hits trillion amid pandemic | McConnell: Chance for relief deal 'doesn't look that good' | House employees won't have payroll taxes deferred MORE (R-Texas) and Senate lawmakers on a weekly basis since he's been confirmed.

The Treasury secretary and National Economic Council Director Gary Cohn met Tuesday night with Brady and other top GOP lawmakers. Mnuchin called the meeting "very successful" and that lawmakers are all in agreement that tax reform is a top priority in order to help boost the economy.

The new White House plan is likely to resemble Trump's campaign tax plan, which was estimated to cost trillions of dollars over 10 years.

A tax-reform bill that increases the deficit could be challenging to pass with just Republican votes under budget reconciliation, since reconciliation bills cannot add to the deficit outside of the 10-year budget window. Tax cuts that lose revenue either need to pass with Democratic support or expire.

Mnuchin said the administration's goal is for a tax bill to be permanent. However, "if we have it for 10 years, that’s better than nothing," he said.

He also said that the White House is focused on tax reform rather than tax cuts, noting that tax reform is "the center and the core" of Trump's economic plan.

Trump has been "very involved" in the tax-reform efforts, Mnuchin said.

The Treasury secretary said that he hopes that Democrats will work with Republicans on taxes.

Mnuchin also touched on the debt ceiling, vowing that it would be lifted later this year.

He said he had already begun talks with Congress and that the issue would not become a political crisis.

"Again, this is a discussion we're having with the House and Senate. I do think we need to raise the debt ceiling" he said.

"There's no question about that. People agree ... we're not going to let this become an issue."

He added: "By the way, we're not going to do one of these wait to the last minutes either. So we'll get the debt limit done in plenty of time."

Mnuchin said he does have the money to manage the government through the summer, and he noted that the Treasury took in a huge amount of revenue on April 18, the deadline for filing last year's taxes.

"We have the cash flow to manage the government. Matter of fact, we had one of the largest days ever of collecting tax receipts on April 18."