WASHINGTON (Reuters) - President Donald Trump’s promised “big announcement” next week on overhauling the U.S. tax code, a top campaign pledge, will consist of “broad principles and priorities,” an administration official said on Saturday.

U.S. President Donald Trump speaks to reporters following a signing ceremony with Treasury Secretary Steve Mnuchin at the Treasury Department in Washington, U.S., April 21, 2017. REUTERS/Aaron P. Bernstein

The president unexpectedly said on Friday at a Treasury Department event that there would be “a big announcement on Wednesday having to do with tax reform.”

In a Twitter message on Saturday, he wrote: “Big TAX REFORM AND TAX REDUCTION will be announced next Wednesday.”

Asked for details, the administration official, who asked not to be identified, said, “We will outline our broad principles and priorities” on Wednesday.

Trump has struggled as president to advance his domestic policy agenda, including on taxes, even though his Republican Party controls both chambers of Congress. With his 100th day in office only a week away, he has yet to offer any formal legislation or win passage of a major bill he favors.

Most recent presidents had legislative wins under their belts by this time in their administrations.

Under U.S. law, only Congress can make significant tax law changes, though the president often drives the tax agenda by offering legislation. The administration official said, “We are moving forward on comprehensive tax reform that cuts tax rates for individuals, simplifies our overly complicated system and creates jobs by making American businesses competitive.”

As a candidate, Trump raised high expectations in financial markets and the business community for changes in the complex, loophole-riddled tax system. In his “Contract with the American Voter,” he vowed to work with Congress on tax legislation “within the first 100 days of my administration.” The action plan promised large tax cuts for the middle class and businesses, a reduction of tax brackets to three from seven, simplified tax forms and an offshore profits repatriation tax holiday.

Since then, no legislation or formal tax plan has been presented by Trump. He has at times expressed support for a plan drawn up by House of Representatives Republicans, but his views are unclear on a section that deals with taxing imports.

In February, Trump promised a “phenomenal” tax plan within a few weeks, without offering details. No plan followed.

Last month when an attempt supported by Trump to repeal the healthcare law known as Obamacare collapsed in Congress, Trump said he would refocus on taxes.

Treasury Secretary Steven Mnuchin said on Thursday he expected Congress to approve a tax plan this year.