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CNN —  

A federal appeals panel grappled on Thursday with arguments presented in a case on the use of Defense Department funds to construct parts of President Donald Trump’s signature border wall.

The 9th US Circuit Court of Appeals expressed concern, in part, over who, if not the plaintiffs, could bring a challenge. Judge Michelle Friedland pressed Justice Department lawyer James Burnham on what Congress could do to stop the government from spending funds.

“I think Congress has a variety of tools to enforce statutes,” Burnham said.

“But wouldn’t someone need to come to the court. I mean, you argued in the DC District Court that Congress can’t come to court, so who stops it?” Friedland asked.

Last month, a federal judge blocked Trump from tapping into Defense Department funds to build parts of his US-Mexico border wall. Although the ruling didn’t prevent the administration from using funds from other sources for the projects, it was a setback for the President on a signature agenda item that has consistently been thwarted by Democrats in Congress.

In the 56-page ruling, Judge Haywood Gilliam of the Northern District of California blocked the administration from moving forward with specific projects in Texas and Arizona, saying Trump couldn’t disburse the funds without congressional approval. The lawsuit that prompted the ruling was filed by the American Civil Liberties Union on behalf of the plaintiffs, the Sierra Club and Southern Border Communities Coalition.

“The position that when Congress declines the Executive’s request to appropriate funds, the Executive nonetheless may simply find a way to spend those funds ‘without Congress’ does not square with fundamental separation of powers principles dating back to the earliest days of our Republic,” wrote Gilliam, a President Barack Obama appointee.

He added that “because the Court has found that Plaintiffs are likely to show that Defendants’ actions exceeded their statutory authority, and that irreparable harm will result from those actions, a preliminary injunction must issue pending a resolution of the merits of the case.”

The decision came more than three months after Trump declared a national emergency to divert billions of dollars from the Pentagon for the construction of his border wall. The proclamation prompted a slew of lawsuits.

The lawsuit brought by the ACLU argues that the declaration was “made solely out of disagreement with Congress’s decision about the proper funding level, location, and timetable for constructing a border wall.” It alleges that construction of the wall would harm the environment and communities along the border.

In February, Trump declared a national emergency to unlock billions of dollars in federal funds to build the wall, bypassing Congress after lawmakers refused to meet his request for the money. Trump and his top officials had argued that there was a crisis on the southern border, though Democrats pushed against that argument, saying there’s no security crisis but rather a humanitarian one.

The decision to invoke a national emergency and bypass Congress was met with bipartisan skepticism. Both chambers of Congress sought to overturn the declaration, but Trump vetoed the measure and the House failed to overturn the veto.

The Democratic-led House of Representatives also sued in an effort to stop Trump from using Defense funds for the border wall. A federal judge denied the request, saying, in part, that the House lacked standing to bring the challenge.

The House has since appealed.