The Trump administration is attempting to reverse decades-old policy to exclude a broad group of immigrants from accessing social services and health benefits, a move that is causing widespread fear among immigrant advocates even as it has yet to take effect.

Immigration groups and health experts say it could harm millions by cutting off access to basic health services, including for people who are U.S. citizens and immigrants who have legal permission to be in the country. They have been pressing the administration for more information, but say there have been few details forthcoming.

“That’s the other thing about these notices, they leave so many questions unanswered,” said Tanya Broder, senior counsel on health and economic justice policy at the National Immigration Law Center.

“They don’t tell people how immigration status will be verified, whose immigration status is relevant,” she added. “They say more guidance is forthcoming, but they don’t name the services that are exempt from restrictions, and they leave people confused on what they should do.”

The administration in July announced it was reinterpreting a 1996 federal law that has allowed immigrants access to certain public benefits, part of an effort to further crack down on what the White House says is the abuse of taxpayer services by undocumented immigrants.

The Department of Health and Human Services (HHS) said it was expanding the definition of “federal public benefit” to include a host of new services that would be off-limits to undocumented immigrants – including Head Start, Title X family planning services, and the community health center program.

“For too long, the government has diverted hardworking Americans’ tax dollars to incentivize illegal immigration,” Health and Human Services Secretary Robert F. Kennedy Jr said in a statement announcing the new policy.

© The Hill