From an Associated Press article by Erica Werner about the upcoming immigration bill:
The test will begin Thursday before the Senate Judiciary Committee. As of Tuesday evening's deadline for filing amendments, some 300 had been offered by Democrats and Republicans. A number of them, particularly from the GOP side, would strike at the heart of the bill in a way supporters say would destroy the fragile agreement between four Republican and four Democratic senators who wrote the legislation. ...
Perhaps the biggest potential trouble for the bill loomed from the Democratic side, in the form of two similar measures filed by Judiciary Chairman Patrick Leahy, D-Vt., to extend immigration rights to gay citizens and permanent residents, allowing them to petition for U.S. residence for their partners just as straight married Americans can.
All four Republican members of the Gang of Eight have voiced serious concerns about such a provision, saying it could kill the bill or cost their support. Those objections are why it was left out of the bill in the first place, despite vigorous advocacy from gay-rights groups.