The man poised to lead India -- one of America's staunchest allies in Asia and a nation of more than a billion people -- cannot even legally enter the United States. 

In what has triggered a quiet battle on Capitol Hill, advocacy groups and lawmakers are drawing attention to the case of India's Narendra Modi. Though the powerful politician is the front-runner in India's elections next year, he was denied a U.S. visa in 2005 over deadly riots in his state. 

This leaves the State Department with a difficult choice -- lift the ban and anger human rights groups while triggering a potential legal battle, or keep the ban and cause a rift with India, one of the United States' closest allies.