President Donald Trump has been trying to rob Congress of its centuries-old powers of the purse, according to legal challenges filed against Democrats against the administration, and that fight will intensify as the executive and legislative branches try to avert a government shutdown.

The administration complicated spending talks by proposing to cancel $4.9 billion of foreign aid using a “pocket rescission” that Democrats and the top Republican Senate appropriator have said is illegal, and congressional Democrats have added that figure to the estimated $425 billion that lawmakers have approved but the president and his officials want to claw back, reported CNN.

“This is a big deal," said Rep. Rosa DeLauro (D-NJ), the top Democrat on the House Appropriations panel. "[Trump] has concentrated all this power in his hands. They are running around illegally stealing taxpayer money, and it is a unilateral partisan act that excludes any cooperation with Democrats.”

Democrats have won some legal challenges forcing budget chief Russ Vought to provide more transparency about what he's doing with legislative spending, but the Supreme Court and federal courts have allowed the administration to cut some foreign aid and research grants at the National Institutes of Health.

“Republicans should not accept Russ Vought’s brazen attempt to usurp their own power," said Sen. Patty Murray (D-WA). "No president has a line-item veto — and certainly not a retroactive line-item veto."

Some congressional Republicans have grumbled about Trump moving to grab funding lawmakers approved, with Sen. Susan Collins (R-ME) calling the moves “a clear violation of the law,” and deficit hawk Thomas Massie, a Kentucky Republican congressman, questioned the pocket recission.

“It looks like they’ve done it intentionally," Massie said. "I think they are trying to be too cute by half by doing it this late. It’s also a risky move. The courts could tie it up. If it’s such a good idea, we have a majority in the House and Senate, why not let us vote on it?”

Trump campaigned on grabbing spending power from Congress so he could “squeeze the bloated federal bureaucracy for massive savings," and the White House budget office defended its own moves as fully legal.

“We are on very firm legal footing and we will be making that case and I think the courts, if they do consider this, will decide along the lines of what we’ve articulated,” a White House official said.

The courts so far have been the primary check on Trump's power grab, with House Speaker Mike Johnson (R-LA) pushing the administration's plans to pause congressionally approved spending to help sell a full-year continuing resolution back in March, but Democrats have challenged some of the budget office's moves that were baked into that bill.

"The full-year continuing resolution, which locks prior year funding levels in place through September 30, included $3 billion in emergency funding that OMB would not allocate, which Democrats argued was illegal," CNN reported.

"OMB went a step further to obscure spending by taking down a public website that discloses what are known as apportionments, which is how agencies are specifically doling out funds," the network added. "In a rare instance of bipartisan pushback against the White House, the four leaders of the appropriations committees sent a letter telling the White House to restore the website."

A federal judge ordered the administration to put that website back online last month, and Democrats insist they will again try to limit Trump's attempts to get around the appropriations process during budget negotiations this month, but they acknowledged that some Republicans had to join their fight.

“We tried to do that last March,” said DeLauro said, adding that Republicans argued that Congress had not authority to rein in the president. “That’s bull----, excuse me. Republicans have been silent. They refuse to stand up and fight back.”