Starting Tuesday, the United States government plans to label migrants ineligible for asylum if they pass through another country while on their way to the U.S.-Mexico border.

At least one immigrant rights group has already promised to sue.

There are exceptions to the new rule. But it generally disqualifies migrants from asylum if they did not seek protection in a third country they traveled through between their home and the U.S. Mexico border.

The American Civil Liberties Union said the rule is illegal, and it will sue quickly.

The American Immigration Council opposes the rule as well. Policy analyst Aaron Reichlin-Melnick said it will likely be challenged under the Immigration and Nationality Act.

“Congress made clear that how someone comes to the United States is irrelevant. Everyone has the right to seek asylum. So I expect that to be the heart of any legal challenge,” he said.

Reichlin-Melnick said migrants who come here after passing through third countries can be barred from asylum but only if they were already firmly resettled in another country.