Senate Judiciary Committee Chairman Patrick Leahy (D-Vt.) has told advocates that he will offer an amendment during the bill markup next week allowing gay Americans to sponsor their foreign-born partners for green cards, just as heterosexual couples can. The measure is likely to pass because Democrats face pressure from gay rights advocates to deal with it in committee, rather than on the Senate floor, where the odds of passage are far less favorable.

But by doing so, Republicans warn that Democrats will tank the whole bill.

“It will virtually guarantee that it won’t pass,” Sen. Marco Rubio (R-Fla.), a member of the Gang of Eight negotiating group, told POLITICO in a brief interview. “This issue is a difficult enough issue as it is. I respect everyone’s views on it. But ultimately, if that issue is injected into this bill, the bill will fail and the coalition that helped put it together will fall apart.”

As the legislation moves through the Judiciary Committee and on to the Senate floor, many people will make pronouncements about things that must be kept in or kept out of the bill — but few issues worry the Gang of Eight as much as same sex partner rights.