FILE - Treasury Secretary Steven Mnuchin listens at right as President Donald Trump speaks during a meeting on the Federal budget in the Roosevelt Room of the White House in Washington. (AP Photo/Evan Vucci, File)

President Trump said he would release new details about his plan to overhaul the tax code on April 26, a sign that he is accelerating one of his most ambitious campaign promises even though key details remain very closely held.

“We’ll be having a big announcement on Wednesday having to do with tax reform,” Trump said Friday while visiting the Treasury Department. “The process has begun long ago but it really formally begins on Wednesday.”

Addressing Treasury Secretary Steven Mnuchin, Trump said, “So, go to it.”

The statement caught many congressional aides - and even some administration officials - offguard, as they thought they were working on a slower timetable. But Trump jolted the process forward on Friday, as he's trying to breathe new life into an effort that risked becoming bogged down like other campaign priorities.

Trump plans a major cut in tax rates, focused on simplifying the tax code for individuals and families, lowering the corporate tax rate and a large tax cut for the middle class. He has also said he wants to create some sort of “reciprocity” tax that imposes a tariff-like tax on imports from countries that have tariffs against the United States.

Earlier Friday, Trump told the Associated Press in an interview that the tax cuts he would propose would be "massive" and perhaps the biggest of all time.

Wednesday’s announcement isn’t expected to be a specific legislative proposal, but the plan is to provide more details than what the White House has issued so far, a senior administration official said.

Mnuchin has worked on the tax plan for months, but details have remained fluid, with White House officials considering a range of options in how they restructure the tax code. White House National Economic Council Director Gary Cohn suggested on Thursday that many of the details were still in flux during comments he made to the Institute of International Finance.

Trump has said a big tax cut will boost economic growth, help companies invest, and lead to more job creation. But Democrats and some Republicans have said any cut in rates should be offset by the elimination of tax breaks to prevent the changes from widening the budget deficit. Mnuchin said on Thursday that the tax cuts would essentially pay for themselves because there would be so much economic growth that it would bring in new revenue to the Treasury Department.

Both Trump and Mnuchin have promised that the overhaul of the tax code they are planning would be the biggest since the Reagan administration.

During the campaign, Trump proposed cutting the corporate tax rate from 35% to 15%, and he also wants to cut the rates individuals and families pay. He has said he wants a big tax cut for the middle class, though many politicians define “middle class” differently.

"People can’t do their returns," Trump said Friday. "They have no idea what they are doing. They are too complicated."

Many congressional aides were caught by surprise from Trump's announcement, as White House officials have expressed that they were working hard on a plan but nowhere near ready to provide new information. Lawmakers have been anxiously waiting for more details of Trump’s plan for weeks.

“I appreciate the President’s leadership and strong commitment to comprehensive tax reform,” House Ways and Means Committee Chairman Kevin Brady (R., Texas) said Friday. “Ways and Means Republicans are ready to work with President Trump and his team on reforms that will grow our economy, create jobs, and increase paychecks.”

The release on Wednesday will come during a critical period for Trump. House Republicans are also working on plans to vote to repeal the Affordable Care Act next week, and Congress must vote by Friday to continue funding the government or there will be a partial shutdown.