President-elect Donald Trump may ask Congress for American tax dollars to pay for a border wall with Mexico, breaking a major campaign promise, according to multiple reports late Thursday

Making Mexico pay for a wall to stop the flow of smuggled drugs and illegal immigrants was a centerpiece of Trump’s presidential campaign.

“Mexico will pay for the wall,” Trump said at a campaign rally in Phoenix on Aug. 31. “Believe me. 100%. They don’t know it yet, but they’re going to pay for the wall.”

Critics have assailed the wall as being unrealistic, expensive and impractical.

According to CNN and the Associated Press, Trump’s transition team has spoken with Republican Congressional leaders about the possibility of funding the wall through the appropriations process, using the authority of a law passed in 2006 under the Bush administration to build fencing along the border. Doing so would avoid having to pass a new border-wall bill, which would likely face heavy opposition by Democrats and many Republicans.

“By funding the authorization that’s already happened a decade ago, we could start the process of meeting Mr. Trump’s campaign pledge to secure the border,” Rep. Luke Messer, R-Indiana, told CNN on Thursday.

Placing the controversial funding measure into an existing spending bill could force a budget showdown with Democrats, who could threaten to shut down the government rather than approve the bill. That could be a politically fraught stance for many Democrats, especially those, such as Senate Minority Leader Chuck Schumer of New York, who supported the bill in 2006.

“If tied to the rest of government funding, it’s much harder for the Democrats to stop, and by the way, I think it’s much harder for Democrats to vote against it if what you’re doing is authorizing funding for an existing law,” Messer later told Politico.

However, if the 2006 law does end up paying for a new border barrier, it will more likely be an upgraded fence, not the enormous concrete wall that Trump touted during his campaign. Experts have said a wall that Trump has described, rising 40 feet and spanning almost 2,000 miles of rugged terrain, would cost tens of billions of dollars to build.