Sixteen Republican-led states are suing to end a federal program that could potentially give nearly half a million immigrants without legal status who are married to U.S. citizens a path to citizenship.

They apply to the Department of Homeland Security, and if approved, have three years to seek permanent residency. During that period, they can get work authorization. The administration estimates about 500,000 people could be eligible, plus about 50,000 of their children.

Before this program, it was complicated for people who were in the U.S. illegally to get a green card after marrying an American citizen. They can be required to return to their home country — often for years — and they always face the risk they may not be allowed back in.