The long-fought Senate immigration bill that opponents say grants amnesty to 10 million illegal aliens is unconstitutional and appears headed for certain demise, Senate Republicans now say. A key feature of the Senate bill is that it would make illegals pay back taxes before applying for citizenship, a requirement that supporters say will raise billions of dollars in the next decade. There's just one problem: The U.S. Constitution specifically prohibits revenue-raising legislation from originating in the Senate.