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Donald Trump opened the door to a potential gas tax hike on Monday as a way to pay for infrastructure improvements — despite the GOP's likely opposition and his team's promise that the building plan wouldn't cost taxpayers.

“It’s something that I would certainly consider,” Trump of a federal gas tax increase in a Bloomberg News interview that underscored just how tough it will be to accomplish the $1 trillion infrastructure investment Trump promised as a candidate.

Democrats typically champion infrastructure investments, but have shown little appetite to cooperate with the Republican president, while Trump's own party historically has opposed the spending or tax hikes needed to fund road and bridge repairs.

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White House Communications Director Sean Spicer was asked about Trump's comments later in the day, and suggested the president was simply considering the idea out of respect to the truckers' industry group.

Trump said in another interview Monday morning that he expected the infrastructure plan to be released in two-three weeks.

Related: Both Parties Say Trump's Infrastructure Plan Needs Repair

The federal gas tax is currently 18.4 cents per gallon and 24.4 cents for diesel and hasn’t been raised in twenty years, leaving the Highway Trust Fund broke. State gas taxes range from 12-58 cents per gallon on top of that.

Advocates of infrastructure investment have called for hiking the gas tax — or tying it to inflation — but Republicans have typically shot the idea down and argued that it hits the middle class. What's more, the memory of fill-ups that cost more than $4 per gallon in the height of the recession have left little appetite in Congress, even though gas prices are down more than a dollar per gallon since then.

Americans for Tax Reform, the lobbyist group run by Grover Norquist, has opposed hiking the gas tax in the past. Armed with a widely-signed lawmaker pledge not to raise taxes that aren't being offset, the group holds enormous power on the issue.

"There is no need — and no excuse for a tax hike," Norquist wrote in a lengthy statement to NBC News suggesting other legislative alternatives to hiking the gas tax, including stopping the diversion of highway trust funds to non-highway projects, and streamlining permitting and impact studies to reduce costs. "We can have more roads at lower prices if Congress repeals destructive laws and rules it itself established for sordid reasons."

Trump and his allies in the GOP have said public-private partnerships would help them accomplish the investment without significant public funding, though few believe it's a fix-all solution and American public-private partnerships typically include significant taxpayer dollars.