Senior White House officials said Friday that the funds President Donald Trump will take from the Pentagon and the Treasury Department to pay for his wall along the U.S.-Mexico border would be “back-filled” in his 2020 budget request.

That means U.S. taxpayers would pay for every penny of the wall in fiscal 2019 — even though Trump long promised that Mexico would pay for it.

Trump is using his executive authorities to move $2.5 billion from the Pentagon’s drug interdiction initiative and $600 million from a Treasury Department drug forfeiture program to fund the wall. With the $1.375 billion for fencing along the southern border included in the spending bill he is expected to sign on Friday, that would bring the total for a barrier to $8 billion.

[Why 19 Democrats and 109 Republicans voted against the government funding deal]

Trump is choosing to use his executive authorities because he concluded Congress is “simply incapable” of funding his proposed border barrier, acting Chief of Staff Mick Mulvaney told reporters on a call Friday morning before the president announced his plan in the Rose Garden.

Democrats, however, have simply refused to fund the kind of concrete — and later steel — structure he has talked about since he launched his presidential campaign in 2015.

“With the $8 billion, we should have enough money to do what we wanted to do with the $5.7 [billion],” Mulvaney said. He also defended the emergency declaration as Democrats and left-leaning groups already are threatening legal action, noting over the nearly 60 national emergencies declared in the modern era, over 30 remain on the books.

[White House: Wall funds would be ‘back-filled’ in 2020 budget request]

“It’s not as if he’s waving a magical wand” to tap a special amount of money, Mulvaney said. “The authority … has been on the books since 1976.”

Asked from which military construction projects will be tapped, a senior White House official said lower-priority initiatives are being studied — but nothing to hinder “lethality.”

Democratic leaders and left-leaning organizations have threatened a court challenge to Trump’s plans.

Watch: Senate leaders interrupt Grassley to announce Trump’s support for deal, national emergency plans

Trump and his top aides in recent days had signaled they were searching through the Office of the President’s legal authorities for ways to shift existing funds into the border barrier project.

“We have options that most people don’t really understand,” Trump told reporters at the White House on Wednesday. He also had teased moving monies from “far less important” programs within the federal budget.

A senior administration official on Friday said there are no restriction from Congress on the $6.6 billion Trump plans to tap from the Pentagon and Treasury.

But in a Thursday statement, Speaker Nancy Pelosi of California and Senate Minority Leader Charles E. Schumer of New York were explicit in threatening a legal fight over his emergency declaration.

[Trump defends signing national emergency to build border wall]

“The Congress will defend our constitutional authorities,” the top Democrats said after Pelosi used her weekly press conference to blast Trump for moving toward what she called an “end-run around Congress,” which controls how federal funds are allocated.

Left-leaning organizations like Public Citizen said Thursday they intend to sue the president soon after he signs the declaration.

Trump’s top spokeswoman, however, brushed off the threat. “We are very prepared, but there shouldn’t be. The president is doing his job. Congress should do theirs,” Sanders told reporters outside her office Thursday.

On the Friday call, a second senior official said Democrats are being disingenuous when they claim they held firm and did not help pass a funding measure for Trump’s wall. The official said the administration long ago agreed to the kind of bollard-style fencing already in place in some areas and which the compromise bill would finance.