What is the Administrative Procedure Act, the law that courts may use to try to slow Trump down?

A nearly 80-year-old federal law, passed in response to a fast-moving, powerful Democratic administration, is now throwing a wrench into Donald Trump’s plans to rapidly slash spending and roll back Biden-era policies.

Many of the Republican’s signature actions so far have faced court challenges, and it doesn’t take long in these complaints to find mention of the 1946 Administrative Procedure Act.

The law passed on a bipartisan basis after the death of President Franklin Delano Roosevelt, who massively expanded the influence of the federal government as he tackled crises including the Great Depression, WWII, and created the New Deal social welfare agenda.

The act requires, among other things, that federal agencies use sound factual considerations for their decisions, seek public comment when making new rules or repealing old ones, and avoid actions that are “arbitrary and capricious.”

At least 30 challengers to the Trump agenda have cited the law, arguing the administration has either skipped these steps on its way to making changes — or has thrown the government into chaos for no clear reason at all.

A lawsuit from government contractors cites the law in its challenge to the Trump administration’s gutting of the U.S. Agency for International Development and pause of foreign aid, arguing the moves “violate basic precepts of administrative law, numerous federal statutes, and bedrock separation-of-powers principles.”

Another cause claims the White House violated the act as it offered thousands of federal employees a buyout if they agreed to leave work and immediately resign later this year, despite not appearing to analyze the costs of such an approach or have clear government funding to support such a measure.

A lawyer representing federal employees accused the administration of “making this up as they are going along,” as the White House appears to be following adviser Elon Musk’s signature approach of rapid staffing changes and deep cuts that he’s used at his tech companies.

"In the tech universe, 'move fast and break things' is a fine motto in part because they're not playing with the public's money, and it's expected that most initiatives are going to fail," Loyola Marymount law professor Justin Leavitt told ABC News about the case. "Congress knows that, so in 1946 they basically said, 'When agencies do stuff ... they have to be careful about it. They've got to consider all aspects of the problem."

The law also came up in a suit from a transgender prisoner challenging the Trump administration’s effective repeal of protections in a federal law aimed at stopping prison rapes, and it was mentioned in suit about another rollback, this time of an immigration directive that previously barred deportation raids at sensitive locations like churches.

The 1946 law halted parts of the Trump agenda during the Republican’s last term, including directives seeking to add a citizenship question to the U.S. census and an attempt to end protections for undocumented “Dreamer” migrants brought to the U.S. as children, but there’s no guarantee the rule will permanently alter things this time around.

The Supreme Court, which already had a conservative majority during Trump’s first term, now has a 6-to-3 conservative supermajority, making the president more likely to prevail on appeals.

Moreover, the administration has suggested it doubts that courts fully have the power to constrain the president, with Vice President JD Vance suggesting on Sunday that federal courts “aren’t allowed” to limit the White House’s “legitimate power,” setting off concerns of a potential constitutional crisis.

Later that day, President Trump said he will always “abide” by court rulings.

However, experts argue that even if courts do put the breaks on Trump, and the administration complies, there’s plenty the government can change before actions are scrutinized in court.

“The Administration is acting so rapidly, and with so little transparency, that considerable damage may be done before anyone outside knows and can get into court,” Georgetown law profess David Super wrote recently in an article for the Center of Budget and Policy Priorities.

It can also respond to court rulings and provide new justifications for actions that were previously challenged.