In a bid to swiftly deliver on one of President-elect Donald Trump's chief campaign promises, congressional Republicans are exploring ways to begin funding a barrier on the U.S. southern border starting as soon as April.

Multiple lawmakers described the plans Thursday, which would use authority under a 2006 law supported by Republicans and Democrats alike to justify spending that could eventually reach into the billions of dollars.

From the beginning of this campaign, Trump pledged to "build a wall and make Mexico pay for it." While the funding under discussion in Congress would be from U.S. taxpayer funds, it would not preclude Trump's administration from seeking reimbursement from Mexico, as Trump himself has discussed on the campaign trail.

"I said Mexico is paying for the wall, with the full understanding that the country of Mexico will be reimbursing the United States for the full cost of such a wall, OK?" he said during an Oct. 22 speech in Gettysburg, Pennsylvania. "We're going to have the wall. Mexico is going to pay for the wall."

A number of Republican lawmakers believe that Trump has authority under the Secure Fence Act of 2006 to commence construction on a wall. That law, backed by President George W. Bush, mandated 700 miles of "reinforced fencing" along the U.S.-Mexico border along with enhanced surveillance systems that came to be known as a "virtual fence." But the full complement of barriers was never completed, and GOP lawmakers believe that the law provides sufficient authority to complete a full border wall like that described by Trump.

That would allow Congress, without passing a new piece of legislation, to start funding the wall through the normal appropriations process. Current federal spending authority expires on April 28, and Republicans could push to include border wall funding in any spending legislation that would follow. While Democrats could well block a separate border wall bill, it would be more difficult for them to block spending legislation, thus risking a government shutdown.

Several high-profile Democrats, including then-Sen. Barack Obama and current Senate Minority Leader Charles Schumer, voted for the Secure Fence Act.

"It's an existing law that hasn't been implemented, in part based on the ideology and philosophy of the outgoing president," said Rep. Luke Messer, R-Ind., chairman of the House Republican Policy Committee, who said he was one of several lawmakers pushing the approach. "I see nothing partisan about trying to comply with existing laws."

The Associated Press first reported on the GOP discussions Thursday.

Key policy decisions have not yet been made - where to start building, for instance, or whether the barrier ought to be a fence or a solid wall. Messer said a broader border security bill could follow a move to start spending on a wall.

"It would be a proposal that would cost billions of dollars to get done, but if it's an appropriate priority for our country, it's worth spending that kind of money," Messer said.

Another lawmaker, Rep. Steve King, R-Iowa, a fierce opponent of illegal immigration said he did not know precisely how Trump wanted to proceed with the wall, but speculated on what his considerations might be.

"I think what he's asking is: What do you have now? What are the assets we have to work with, and what are the challenges for right-of-way acquisition?" King said. "What are the tools they have within the departments? How much money is sitting around in the [Department of Homeland Security] that could be reallocated? What do they have in the books for design, and how much engineering have they done? He's got the authority to do a lot without moving legislation, but he wants to know what would require legislation."

King said he was not especially concerned that Congress, not the Mexican government, would be writing the first checks.

"If we build that wall, and Donald Trump hasn't figured out how to get Mexico to pay, I'm not going to be the guy who says, 'Let's wait until we get this in pesos,' " he said.

The Washington Post's David Weigel contributed to this report.