U.S. Senate Judiciary Committee Chairman Patrick Leahy on Tuesday proposed a gay rights amendment to the Senate's immigration bill, prompting one of the measure's Republican sponsors to repeat his prediction that it could sink the legislation. "It'll kill the bill," Florida Senator Marco Rubio said in a brief interview. "There is a coalition of groups who are supporting immigration reform who will not support it if that's in there." Rubio, the son of Cuban immigrants and a possible Republican presidential candidate in 2016, is key to attracting conservative support for the bill. It remained uncertain whether the amendment would make it through the committee, which is controlled by Democrats.