WASHINGTON – Republicans on Capitol Hill say they don't need to wait for Mexico to make good on President-elect Donald Trump's central campaign promise: building a southern border wall.

In fact, they are happy to underwrite the wall themselves, at a potential cost of many billions of dollars.

The GOP's willingness to fund Trump's border wall with taxpayer money could put the party's deeply held desire to rein in government spending in conflict with its long-standing goal of cracking down on illegal immigration and toughening border security. Nonetheless, many Republicans do not see an inherent conflict.

"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," said Rep. Luke Messer, R-Ind., chairman of the House Republican Policy Committee.

There is no reliable price tag on building a border wall, but Trump has estimated the cost at $8 billion. Recent congressional legislation pegged the number at $10 billion, and construction experts say it could be more than double that.

The wall is one part of a massive spending strategy at the core of Trump's populist agenda. Trump has not provided extensive details on how he plans to follow through on vows such as overhauling the tax code, repealing and replacing the Affordable Care Act, and executing a sprawling infrastructure program. But each of those proposals carries an exorbitant price tag, and experts say that combined, they could add trillions of dollars to the deficit.

Experts at the nonpartisan Tax Policy Center estimate that Trump's tax proposals alone could add as much as $7.2 trillion to the deficit in the coming decade. Even-more-conservative estimates, such as those produced by the right-leaning Tax Foundation, concluded that Trump's tax proposals would create at least a $4.4 trillion budget hole in the same time frame.

The costs of rolling back the ACA are harder to predict. Repealing the entire law would increase deficits by more than $350 billion over a decade, according to a 2015 report from the Congressional Budget Office, which serves as an independent scorekeeper.

If Congress plans to mimic a 2015 attempt at repeal, the eventual legislation could reduce deficits by $282 billion. But lawmakers have yet to reveal a plan to replace the ACA with tax breaks and benefits that could more than offset any savings.

Trump has not outlined a detailed plan for infrastructure spending. But his nominee to be commerce secretary, Wilbur Ross, has suggested that the government offer approximately $140 billion in tax credits to nudge companies to invest $1 trillion of their own money.

Under President Obama, Republicans decried new federal spending to stimulate the economy, expand health-care coverage and pursue other domestic priorities. But so far, they seem to harbor no similar qualms about Trump's platform.

"I think realistically we're going to have to find a way to fund this," Sen. Steve Daines, R-Mont., an Appropriations Committee member, said about the border wall. He said the Trump transition team has not directly contacted the committee on the issue.

Other Republicans reacted with a shrug, pointing to support and even money for building a wall that has already been provided by Congress.

"We've already appropriated money for walls," said Sen. Charles Grassley, R-Iowa. "We've got walls right now."

Rep. Steve King, R-Iowa, a fierce opponent of illegal immigration, is not worried about when or how Mexico reimburses the U.S. government for the massive construction project.

"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,' " King said.

The Trump team is committed to moving forward on the wall quickly and before Republicans become uneasy about the project and its political cost, according to several people close to Trump.

A national exit poll of presidential voters found that only 41 percent supported building a wall along the entire border with Mexico. But among Trump supporters, three-quarters backed the proposal.

And there is plenty of skepticism that Trump will ultimately follow through: A CNN/ORC poll taken after the election found just under half of all Americans thought it was at least somewhat likely that Trump would build a wall along the southern border, while a November Quinnipiac poll found just 19 percent of registered voters thought Trump would get Mexico to pay for such a wall.

Trump's strategy is being driven by several advisers, in particular those with political links to Sen. Jeff Sessions, R-Ala., Trump's nominee to be attorney general. They include speechwriter and policy aide Stephen Miller, incoming deputy White House chief of staff Rick Dearborn and incoming White House chief strategist Stephen Bannon.

Sessions himself, who has focused on border security for decades, also is intimately involved in the discussions. Miller is already drafting executive actions related to border and immigration policy, and Dearborn is working with Republican leaders to coordinate legislation.

Bannon, the people close to Trump added, is paying close attention to officials he may be able to work with inside the Mexican government on the border and other issues, with new Mexican Foreign Minister Luis Videgaray familiar to him and other Trump aides. Videgaray was involved in Trump's trip to meet Mexican President Enrique Peña Nieto, which sparked controversy during the campaign.

Vice President-elect Mike Pence, who once served in the House and is friendly with many Republicans still skittish about Trump, is considered key to smoothing any tensions as the push to build the wall gets underway.

During the campaign, the wall was Trump's signature promise and a raucous rallying cry among his supporters at his large rallies. Trump repeatedly said that Mexico would have to pay for the giant structure, but he also signaled that U.S. taxpayers would be reimbursed for it after its construction.

In a Friday-morning tweet, Trump said: "The dishonest media does not report that any money spent on building the Great Wall (for sake of speed), will be paid back by Mexico later!"

Republicans are discussing reviving a 2006 law – supported by the GOP and Democrats alike – that gave them authority to construct partial fencing along the 2,000-mile U.S.-Mexican border.

The 2006 Secure Fence Act mandated 700 miles of "reinforced fencing" along the southern border as well as enhanced surveillance systems. The full complement of barriers was never completed, and GOP lawmakers think the law provides sufficient authority to proceed with additional construction.

That would allow Congress, without passing new legislation, to start funding the wall through the normal budget appropriations process. Current federal spending authority expires April 28, and Republicans could push to include such funding in any spending legislation to follow. The plan may put Democrats in a political bind, forcing them to advocate shutting down the government if the wall is included in a must-pass spending bill – mirroring a similar position in which Republicans frequently found themselves during the Obama years.

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

House Minority Leader Nancy Pelosi, D-Calif., dismissed Republicans' invocation of the bipartisan 2006 law – "That's about politics not about policy," she said – and said funding for a wall would be a "heavy sell."

Pelosi also asked where the money would be generated.

"What is this coming out of? . . . Is this coming out of education? Is this coming out of everything we do on the domestic side?" she asked, while mocking Trump's notion that Mexico would eventually pay for it: "Oh, yeah, really?"

Some lawmakers have shied away from committing to building a continuous wall along the border, and some think it could be constructed in different ways and out of a variety of materials, to include fencing.

"I think it could take several different forms," Sen. Mike Rounds, R-S.D., said Friday.

Sen. Ron Johnson, R-Wis., chairman of the Senate Homeland Security and Governmental Affairs Committee, said Friday he had been impressed with a system of fences he had inspected along the Israeli border with Palestinian territories.

"I'm always looking for best practices," he said. "It's been incredibly effective. They had thousands of illegal immigrants; it's down to the teens."

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