President Donald Trump gestures as he arrives for the World Economic Forum (WEF) annual meeting in Davos, Switzerland January 25, 2018.

President Donald Trump's massive infrastructure package just hit a major roadblock.

Prominent Republican lawmakers are already coming out against raising the federal gas tax to pay for the president's promised $1 trillion investment in infrastructure. Speaking on Saturday night at a private donor retreat here hosted by billionaire industrialists Charles and David Koch, Senate Majority Whip John Cornyn opposed the idea.

"I'm not for raising the gas tax," he told the roughly 500 attendees. "It's going to be a declining source of revenue."

The Trump administration is preparing to release an infrastructure plan in the coming weeks that reportedly includes at least $200 billion in federal spending that would jumpstart investment from the private sector, and state and local governments.

Yet the proposal is not expected to outline where the money would come from, leaving Congress to fill in the details.

The U.S. Chamber of Commerce unveiled a plan earlier this month to raise the gas tax by 25 cents — five cents a year for five years — a move the group acknowledged would be an uphill battle. The chamber estimated it would cost drivers $9 a month and raise $394 billion over the next decade.