WASHINGTON (AP) — Business leaders began an aggressive lobbying effort Thursday to ensure that their vision for overhauling the U.S. tax system isn't lost in the chaos consuming President Donald Trump's administration.

The Koch brothers' political network announced it is preparing to spend millions of dollars to promote a tax overhaul.

That word came as a handful of business executives told a congressional committee that the current tax system makes U.S. companies uncompetitive.

"We no longer live in a world where the U.S. can set a corporate tax rate without considering what our international competition looks like," John Stephen, AT&T's chief financial officer told the House Ways and Means Committee. "Countries are vigorously competing against each other to attract investment and jobs, but the U.S. has done little to retain its competitive advantage."

Billionaire industrialists Charles and David Koch are undertaking a multimillion-dollar campaign through the summer to ensure their conservative tax plan is not forgotten, said James Davis, spokesman for the Kochs' political network.

The campaign will include digital ads and town hall meetings, along with phone banks and direct mail.

"Now is the time. We've got to unite around these principles," Davis said. "The White House hopefully will see this as a jolt to support them in driving this forward."

The Koch push reflects broader concerns from the nation's business community that Trump's promise of a tax overhaul may fall victim to his mounting political challenges. The stock market on Wednesday suffered its largest single-day loss of the Trump presidency. That was before the Justice Department appointed a special counsel to investigate allegations that Trump's campaign collaborated with Russia to sway the 2016 election.

Treasury Secretary Steven Mnuchin said 100 people at his department are working on an overhaul, and that the goal is to bring "meaningful relief" to the middle class and make American businesses competitive.

Mnuchin testified before the Senate Banking, Housing and Urban Affairs Committee — his first appearance before that committee since being sworn in.

He said the U.S. can achieve 3 percent economic growth if the United States makes historic changes in taxes and regulations. The Treasury Department also is preparing a report on regulatory changes for the finance system. Mnuchin said it will be broader than simply undoing much of the Dodd-Frank law enacted under President Barack Obama.