U.S. District Judge Sharon Blackburn wrote last month that she would issue a ruling on the law no later than Wednesday. Her order delaying enforcement of the law expires Thursday.
Among other things, the new law would ban anyone from helping an illegal immigrant remain in Alabama. It also requires that schools verify the citizenship status of students.
Republican supporters of the law say it's designed to save jobs for legal residents. The Obama administration, immigrant-support groups and others claim the law is overly harsh and violates the Constitution.
Federal courts already have blocked all or parts of less-restrictive laws in Arizona, Utah, Indiana and Georgia.