Senate Majority Leader Harry Reid (D-Nev.) on Tuesday warned that a Republican amendment to strengthen border security would be a "poison pill" that could kill the overall immigration reform bill.
The Senate Tuesday voted to advance debate on immigration reform, bringing the contentious issue to the upper chamber floor for the first time in years.
The four GOP members of the Gang of Eight, which crafted the bill, were joined by more than 20 other Republicans in the vote. But those members have cautioned that they are unlikely to support the final bill unless it includes tougher measures to tighten border security.
An amendment sponsored by Sen. John Cornyn (R-Texas) is seen as crucial to winning their support. The measure would require the government to meet border enforcement goals before granting full legal status for the nation's estimated 11 million immigrants in the country illegally.
Under the amendment, an immigrant already in the country would not be able to obtain legal permanent residency until the government had 100 percent awareness of border activity and was able to intercept at least 98 percent of illegal crossings.
The measure also calls for implementation of a biometric system for tracking all visa holders and establishing an E-Verify employee check system as preconditions.
Reid on Tuesday called those conditions unacceptable.
"He has set out the principles of what's in that amendment, and his principles would be a poison pill to this bill," Reid said.
Cornyn said that he has been talking to the Senate Gang of Eight members and is trying to gain the support of Sen. Marco Rubio (R-Fla.). Rubio, a Tea Party favorite, is seen as key to winning the backing of conservatives.
"I've talked fairly regularly with Sen. [Charles] Schumer [(D-N.Y.)] and conversations continue," Cornyn added, referring to another member of the bipartisan immigration group.
"I think if they had 60 votes to pass the bill out of the Senate they probably wouldn't be talking to me," he added. "Which tells me they view this as a way to get it out of the Senate on a bipartisan basis that would give it some momentum and increase the likelihood of the bill passing in the House as well."
Senate Minority Leader Mitch McConnell (R-Ky.) called Cornyn's measure "the key amendment" in the immigration reform debate.
"It will put us in a position where we can look the American people in the face and say we are going to secure the border," he said. "It will be a very, very important amendment."
McConnell said that Sen. Orrin Hatch (R-Utah) will also offer an amendment to deny undocumented immigrants without permanent residency the ability to receive any means-tested government benefits.
"There are certainly conditions under which I can support an immigration bill and we'll find out in the course of the next three weeks whether this becomes a bill that I and others will be comfortable supporting," McConnell said.