WASHINGTON — As a senator, Jeff Sessions became Congress’ leading advocate not only for a cracking down on illegal immigration, but also for slowing all immigration, increasing mass deportations and scrutinizing more strictly those entering the U.S. As attorney general, he’d be well positioned to turn those ideas into reality. Immigration laws are enforced by other agencies, but the Justice Department plays a crucial role in setting the policies and legal underpinnings that shape the system. And if Donald Trump sticks with his campaign promises, immigration will be a top priority for his administration.