Even with such overwhelming public support for gay rights, the inclusion of an amendment to allow same-sex couples to sponsor their foreign-born spouses’ green cards continues to remain a hot-button issue for lawmakers.

Sen. Patrick Leahy, D-Vt., followed through on his plan Tuesday to file an amendment to provide that right to gay and lesbian couples. However, critics like Sen. Marco Rubio, R-Fla., who helped draft the reform bill, said that including the same-sex measure in the final bill is setting up immigration reform for failure.

“This is not a blanket bill that covers every immigration scenario,” Rubio said on “The Hugh Hewitt Show” in April. “This immigration bill is difficult enough as it is. There are already enough questions being asked, questions that need to be answered, legitimate points that are being raised.

“You inject something like this in the bill, it will die,” he continued. “The coalition behind it will fall apart and it will not pass. It’s just that simple. If that issue is injected into this bill, this bill will fail. It will not pass. It will not have the support; it will not have my support and so I hope we can avoid this.”

While the amendment is likely to make it through the Senate Judiciary Committee, which begins its markup Thursday, due to the 10-8 Democratic advantage, it could be dead on arrival in the full Senate.

That’s because if someone like Rubio should withdraw his support for the immigration reform bill, Democrats will have a tougher time selling the concept to Republicans, especially those who are more conservative.

The co-sponsors of the bill are hoping to garner 70 votes in order to show House Republican critics that the bill has strong bipartisan support. Only 60 votes are needed to prevent a filibuster or blocking debate on the bill. Should the “gang of eight’s” legislation pass the Senate with more than 60 votes, it sends a clear message to the House, one that says it will need to try and reconcile its immigration reform bill with the Senate’s.

If Leahy’s amendment to provide the proposed rights to same-sex couples succeeds, the question then becomes how many Senate Republican votes are ultimately needed to pass the 2013 immigration bill, said Tamar Jacoby, president of ImmigrationWorks USA.

“I think that is going to be the high drama,” she said. “If you add that amendment it’s not even clear to me that you will get to 60.”

And if lawmakers meet the required 60-vote threshold, it signals to House Republicans, who are already hesitant about reform, to back away.

“In order for it to be appealing to Republicans in the House, you need to get a pretty good vote out of the Senate,” Jacoby said. “There could be a lot of domino effect from this, so to speak.”

That main bill could fail -- Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., expressed concern about the amendment's inclusion -- and could hinder Rubio’s re-election chances. The Florida candidate is a possible presidential candidate in the 2016 elections. Success or failure on this issue could surely return to haunt him three years from now. Not only because Rubio, a Tea Party favorite, is being watched by his conservative constituents for what he supports and what he doesn't, but for the fact that he's at the helm of a major national issue, and it's a big risk.

“He’s already way out on limb on immigration,” Jacoby said. “He has to get re-elected. … I think that amendment could significantly undermine Republican support. I don’t know if it will be so severe that it would mean the bill wouldn’t come out of the Senate. I don’t know, but I certainly think the risk could be that it would come out of the Senate with such anemic Republican support that sends a very troubling message to the House where things are going to be difficult enough without that.”

Ira Mehlman, spokesman for the Federation for American Immigration Reform, or FAIR, which doesn’t support the comprehensive immigration reform bill, said one of the problems with the legislation is that it was crafted behind closed doors with people sharing the same particular view.

“If you look at this bill it is basically amnesty for the people who are here illegally, all sorts of benefits for various interests and lobbies,” he said. Mehlman also said the amendments are the first attempt to get what he calls “meaningful input” into the legislation. Still, he believes the most meaningful input will have to come from the Senate floor.

“I think it is the objective of certainly Leahy and the gang of eight [is] to have minimal scrutiny, minimal number of amendments offered and accepted,” Mehlman said. “That’s been their plan all along. … I think there is certainly legitimate concern on the part of everybody out there that this bill is not crafted in any way that does anything positive for the American people. There is ample reason for people to have very significant reservations about many parts of the bill -- pretty much all of the bill.”