The Trump administration is expected to issue a proposal in coming weeks that would make it harder for legal immigrants to become citizens or get green cards if they have ever used a range of popular public welfare programs, including Obamacare, four sources said.

The move, which would not need Congressional approval, is part of White House senior adviser Stephen Miller’s plan to limit the number of migrants who obtain legal status in the US each year.

Details of the rulemaking proposal are still being finalized, but based on a recent draft seen last week and described to NBC News, immigrants living legally in the US who have ever used or whose household members have ever used Obamacare, children’s health insurance, food stamps and other benefits could be hindered from obtaining legal status in the US.

Immigration lawyers and advocates and public health researchers say it would be the biggest change to the legal immigration system in decades and estimate that more than 20 million immigrants could be affected.

They say it would fall particularly hard on immigrants working jobs that don’t pay enough to support their families.