A watchdog group has already filed a lawsuit regarding President Donald Trump's declaration of a national emergency to build a border wall.

The group is seeking to receive documents from the Justice Department related to the preparation of the declaration.

A government watchdog group in Washington, DC, has already filed a lawsuit related to President Donald Trump's declaration of a national emergency to build barriers on the United States' border with Mexico, becoming the first of what is likely to be many legal challenges to the president's unilateral action.

The group, Citizens for Responsibility and Ethics in Washington, filed the lawsuit Friday, shortly after Trump's announcement. It is demanding that the Department of Justice's Office of Legal Counsel "provide documents concerning the legal authority of the president to invoke emergency powers."

Read more: IT'S OFFICIAL: Trump declares national emergency to build his border wall

"Americans deserve to know the true basis for President Trump's unprecedented decision to enact emergency powers to pay for a border wall," CREW Executive Director Noah Bookbinder said in a statement. "We're suing because the government has so far failed to produce the requested documents or provide an explanation for their delay."

CREW said its lawsuit seeks to have the Justice Department honor the group's Freedom of Information Act request in January for documents showing "relevant communications, including legal opinions, from OLC, the Department of Homeland Security, and the Department of Defense" as they relate to Trump's previous threats to declare a national emergency.

The emergency declaration is expected to face a litany of legal challenges

Shortly after CREW filed its lawsuit, the American Civil Liberties Union announced its intent to challenge the emergency declaration, which seeks to redirect certain appropriated funds to build an extension of the border wall.

"By the president's very own admission in the Rose Garden, there is no national emergency. He just grew impatient and frustrated with Congress, and decided to move along his promise for a border wall 'faster,'" ACLU Executive Director Anthony Romero said in a statement. "This is a patently illegal power grab that hurts American communities and flouts the checks and balances that are hallmarks of our democracy."

Romero said the ACLU "will be filing a lawsuit early next week."

"As the country's premier defender of civil liberties and civil rights, the ACLU will always fight to ensure a robust system of checks and balances on the power of the executive, which is critical to safeguarding our democracy and defending rights," he added.

The ACLU said its lawsuit would argue that Trump's emergency declaration is illegal because "Congress restricted the use of that power to military construction projects, like overseas military airfields in wartime, that 'are necessary to support' the emergency use of armed forces."

In a Friday press conference, California's governor, Gavin Newsom, and its attorney general, Xavier Becerra, also announced their intent to file a lawsuit against the Trump administration.

Newsom said that the lawsuit would take some time and that it would likely be joined by other states.

Early Friday, ABC News reported the Justice Department had warned the White House that Trump's action could be temporarily blocked by the courts.

Other prominent lawmakers — even those supporting Trump — have also questioned the president's legal authority to follow through with the declaration.

Read more: Here's what you need to know about the border-security compromise that could avert another government shutdown

But in a conference call with reporters before Trump's announcement, the acting White House chief of staff, Mick Mulvaney, brushed aside concerns that what the president was doing was unprecedented.

"There's been some concern in the media about whether or not this creates a dangerous precedent," Mulvaney said. "It actually creates zero precedent. This is authority given to the president in the law already. It's not as if he just didn't get what he wanted so he's waving a magic wand and taking a bunch of money.

"I saw Nancy Pelosi said yesterday this sets a precedent for the Democrats to declare a gun emergency the next time they're in the Oval Office. That's completely false," he added. "If Democrats could've figured out a way to do it, they would have done that already."