WASHINGTON — Republicans in Congress are working with Donald Trump’s transition team on a strategy for building the southern border wall that would put taxpayers on the hook for billions of dollars in costs — which the president-elect promises will eventually be borne by Mexico.

Members of Congress and aides said Friday they hope that a funding plan under study would enable construction of the wall to begin early in the Trump administration, which begins Jan. 20.

The plan, relying on authority in a 2006 law, tentatively calls for including construction money in legislation that needs to pass by April to keep the federal government running.

“One way to get started in fulfilling Mr. Trump’s promise would be to fund existing law,” said U.S. Rep. Luke Messer, R-Ind., chairman of the Republican Policy Committee.

“I don’t know that anyone expected the government of Mexico to write a giant check to the U.S. Treasury,” he said.

Trump has estimated the cost at between $8 billion and $12 billion for an “impenetrable” precast concrete wall spanning hundreds of miles along the border.

Initial response to the plan presages the pitched debate in Congress over paying for the wall, which was a rallying point in Trump’s campaign and one of his most divisive proposals.

U.S. Rep. Henry Cuellar, D-Laredo, called the proposed plan evidence of “another broken promise by Trump.”

“Incredible,” was the reaction by U.S. Rep. Lloyd Doggett, D-San Antonio. Trump “has abandoned the ‘Mexico will pay for it now approach’ and is asking American taxpayers to come up with the money.”

U.S. Rep. Blake Farenthold, R-Corpus Christi, is the Texas regional representative of the Republican Policy Committee, which considers GOP legislative priorities. Farenthold said that with new technologies and better enforcement of immigration laws, a border wall might prove to be unnecessary.

But he added: “With the campaign promise Trump made, if he wants to fulfill it and if we can find the money to finance it until Mexico pays for it, there’s no reason not to do it.”

Trump on Friday used Twitter to blast reports of the funding plan, writing that “the dishonest media does not report that any money spent on building the Great Wall (for the sake of speed) will be paid back by Mexico later!”

Trump spokesman Sean Spicer played down coverage of the discussions, reading from a Trump statement in October telling voters not to worry about the wall’s cost because “the country of Mexico will be reimbursing the United States.”

Spicer acknowledged “an ongoing discussion with Congress about how to fund it and obviously the logistics to make it work.” He added that some of the construction probably could begin “right away.”

Republicans say they hope to rely on existing authority under the 2006 Secure Fence Act, signed into law by President George W. Bush, for 670 miles of border fence, including nearly 56 miles across the Rio Grande Valley.

The wall likely will remain a flash point in relations between the United States and Mexico.

Mexico has said it won’t pay for a wall that should not be built. Officials at Mexico’s embassy in Washington did not respond Friday to questions about Trump’s latest assertions.

On Thursday, former Mexican President Vincente Fox wrote on Twitter: “Trump may ask whoever he wants, but still neither myself nor Mexico are going to pay for his racist monument.”

Secretary of State John Kerry on Friday cast further doubt on reimbursement in an interview with ABC.

“They’re not going to voluntarily pony up and pay for something they disagree with,” Kerry said.

Trump and Republicans have spelled out methods to extract monies if needed, including impounding remittance payments and increasing fees on visas and border crossing cards.

On Friday, Messer introduced legislation aimed at curbing the use of child tax credits by undocumented immigrants. He referred to a U.S. Treasury inspector general report estimating the cost to taxpayers at $4.2 billion annually.

The 2011 report found an increasing number of undocumented immigrants, even though they lack Social Security numbers, are using individual taxpayer identification numbers to file tax returns in which they claim an Additional Child Tax Credit, a refundable credit intended for working families.

“The payment of federal funds through this tax benefit appears to provide an additional incentive for aliens to enter, reside and work in the United States without authorization,” the report concluded.

Messer said his legislation aims to close that loophole. Republicans have tried and failed before to block unauthorized workers from getting the refunds.

But until money can be recouped, Messer said, Congress should pay for the wall.

“It’s a big-dollar project, but it’s a matter of priorities,” he said. “This is a high priority for this country and we should make the sacrifices to fund it.”