Republican leaders said Thursday they plan to pony up $12 billion to $15 billion in the coming months to begin construction of President Donald Trump’s proposed border wall, but that large sum of money may be just the first installment to fund a project that could cost taxpayers as much as $40 billion, according to some independent estimates.

In addition, Republicans face likely opposition not just from Democrats but also centrist Republicans and fiscal hawks who would balk at seeing that kind of tab added on to the deficit. Travis Hall, a spokesman for the House Republican Study Committee, said the conservative group will insist on offsets, as it has with supplemental appropriations in the past.

And trying to take the money from other domestic accounts would provoke howls of pain from advocates for those programs, from both parties.

At a joint Republican retreat, House Speaker Paul D. Ryan of Wisconsin and Senate Majority Leader Mitch McConnell of Kentucky told reporters that they would pass a supplemental appropriations bill for the current fiscal year, which ends Sept. 30, to jump start work on the wall along the U.S.-Mexico border. Trump signed an executive order Wednesday ordering Homeland Security officials to begin drafting plans and identifying available funds for the project, as well as putting together a budget proposal to present to Congress.

A final price tag will likely remain murky until the administration hammers out final details, like what material they’ll use to construct the wall; who will build it; and what portions of the Southern border it would cover.

The ultimate cost — which Trump still insists will be reimbursed later by the Mexican government — could be far greater than the $15 billion that GOP leaders plan to pass before October.

Rough estimates

The government has already spent more than $7 billion to build about 653 miles of border fencing along the nearly 2,000-mile border with Mexico, according to a 2016 study from Bernstein, an equity research firm based in New York. A concrete wall would be even more costly and more difficult to build, especially in areas with rough terrain.

Bernstein estimated a 1,000-mile wall, 40 feet tall and 10 inches thick, would cost up to $25 billion.

The MIT Technology Review estimated in October that the cost of building 1,000 miles of wall would be even higher — between $27 billion and $40 billion, based on specs Trump mentioned during the campaign.

For a solid wall 1 foot thick and 50 feet high, with an extra 15 feet underground to prevent tunneling, the cost of concrete and steel alone would top more than $13 billion, the MIT review found. Labor costs would add at least another $15 billion to the tab.

Those huge numbers could scare away lawmakers on both sides, unless Trump reveals a substantial plan to make Mexico pay for the wall, as he promised.

On Thursday, Trump suggested a tax overhaul could “generate revenue from Mexico that will pay for the wall, if we decide to go that route,” an idea that has previously been mentioned by Ryan.

White House Press Secretary Sean Spicer told reporters later that Trump would seek to impose a 20 percent border tax on imports from Mexico to help pay for the wall, but he offered few details how that would work. Spicer estimated the U.S. could generate an additional $10 billion per year and “easily pay for the wall just through that mechanism alone.” He also said the White House was working closely with Congress on such a plan.

And if U.S. taxpayers have to foot the bill, lawmakers would have to either add the spending to the deficit or make significant cuts to other nondefense programs like health research, law enforcement, veterans programs, housing assistance, education grants, other infrastructure projects or any number of other domestic purposes.

On the other hand, Sen. John McCain said that the idea of building a wall along the border with Mexico should not, as a practical matter, be taken literally.

“My general reaction to the ‘wall’ is that border security can be achieved not through just building a wall. Walls can be pierced. Walls can be climbed over. If you ever go to San Diego or even Nogales, Ariz., you’ll see tunnels that we uncover on a weekly basis,” the Arizona Republican told reporters in Philadelphia.

Remember Zika?

Even passing the initial down payment is no small task. Trump and GOP leaders will face near unanimous opposition from Democrats and potential defections from Republican fiscal hawks and centrists.

Supplemental spending bills have become nearly impossible for Congress to pass. In 2016, lawmakers spent more than half the year battling over a relatively tiny amount of money to combat the Zika virus, an issue that both sides agreed needed to be addressed. A massive border wall, which most Democrats view as a waste of taxpayer money, would be far more divisive.

Democrats plan to make Republicans feel the political heat if they don’t offset the cost of the border wall — or somehow get Mexico to pay for it.

“The same Republicans who howled ‘fiscal responsibility’ when it comes to investments to help working families are apparently willing to light billions of taxpayer dollars on fire and add to the federal deficit in order to build Trump’s useless border wall,” said Drew Hammill, a spokesman for House Democratic Leader Nancy Pelosi of California.

More problematic for Trump, Ryan and McConnell would be GOP defections. Some moderate Republicans have already voiced opposition to the wall, like Rep. Will Hurd, who said the project “would negatively impact the environment, private property rights, and economy” in his West Texas border district.

“The facts have not changed. Building a wall is the most expensive and least effective way to secure the border,” Hurd said in a statement Wednesday.

Fiscal hard-liners could also pose headaches for the administration and GOP leaders. On Thursday, Ryan wouldn’t commit to offsetting the new spending with cuts elsewhere in the federal budget, which has been a prerequisite for Republicans to support supplemental appropriations in recent years.

“As far as the offset, we’re going to wait and see from the administration what their supplemental looks like,” he said. “I’m not going to get ahead of a policy and a bill that has not been written yet.”

Rep. Mario Diaz-Balart of Florida, a senior GOP appropriator who oversees transportation infrastructure spending, told CQ he would be “exceedingly reluctant — and that’s an understatement — to do things that increase the deficit on a long term basis.”

There are other routes for Congress to supply the border wall funding  that would draw from a finite pool of money without adding to the deficit. But Trump wants to fast-track the project, and GOP leaders in Congress appear eager to help him quickly make good on his signature campaign promise.

Dean DeChiaro, Niels Lesniewski, Lindsey McPherson and Kellie Mejdrich contributed to this report.