ASSOCIATED PRESS Ivanka Trump spoke against a rule that would require companies to report pay broken down by gender, race and ethnicity.

A federal judge issued a strong rebuke to the Trump administration Monday night.

In a decisive ruling, Judge Tanya Chutkan, who sits on the U.S. District Court for the District of Columbia, found the administration’s attempt to stop an Obama-era equal pay regulation was illegal.

Efforts to stop the rule from going into effect were “arbitrary and capricious,” Chutkan ruled.

The rule requires companies with more than 100 employees to report how much they pay their workers — broken down by gender, race and ethnicity — and would be a crucial advancement in rooting out discriminatory pay practices and narrowing the gender pay gap. Pay data would not be reported on an individual level, but instead broken out by job category and pay bands.

The ruling offers a bright spot for those challenging the Trump administration’s efforts to roll back regulations in a lot of areas, including civil rights, health care and the environment, said Maya Raghu, director of workplace equality at the National Women’s Law Center, which brought the suit along with The Labor Council for Latin American Advancement (LCLAA) and Democracy Fwd.

“The court’s decision sends a powerful message that the Trump administration is not above the law,” she said.

Back in 2017, as companies were gearing up to start reporting this data, the Office of Management and Budget (OMB) issued a stay, claiming that the rule was too onerous for companies and didn’t adequately address privacy and confidentiality concerns.

“Ultimately, while I believe the intention was good and agree that pay transparency is important, the proposed policy would not yield the intended results,” Ivanka Trump said at the time.

Judge Chutkan said the OMB offered no support for its claims that the new rule was burdensome, pointing to the Equal Employment Opportunity Commission’s careful efforts in rolling out the new rule.

The administration could still appeal, Raghu said, but given the thoroughness of Chutkan’s ruling, they’d have a tough time.

With the green light from this ruling, companies could start reporting this data to the EEOC as early as May. The agency could use the information down the line to enforce equal pay laws.

The pay gap in the United States has hardly budged in recent years. Currently women make on average just 80 percent of what men earn, with the gap far wider for women of color.

But efforts to equalize pay have taken off ― with investors increasingly pushing big public companies to report pay information, and some companies actually undertaking efforts publicly to fix the problem.

Earlier this year, under pressure from activist investors, Citigroup voluntarily revealed that the median pay for women is 29 percent less than men’s median pay ― mainly because men hold more senior roles at the big bank.

Other countries are also leading the charge in requiring businesses to report pay data. Companies in the U.K. must now publicly report their gender pay data ― and it’s often not pretty. But as companies are forced to look closely at their numbers, they’re often spurred to change.

“It’s past time for the U.S. to catch up,” said Raghu.