The key difference between President Obama's immigration reform plan and the one proposed by the "Gang of Eight" senators is a so-called "trigger" requirement that says that the government must show evidence of improved border security before illegal immigrants would be allowed a pathway to U.S. citizenship.
The trigger requirement could be a critical stumbling block between Republicans and Democrats
The Republicans argue that conservative members of their party won't accept comprehensive immigration reform without a guarantee that the flow of illegal immigrants across the border has stopped. That means tying the legislation to a trigger that sets specific enforcement goals.
Democrats, meanwhile, that that holding up reform over the trigger could mean that the entire plan gets lost in a legal limbo for the 11 million undocumented residents already in the country. Measurements of improved border security can be highly arbitrary, after all.
Sen. Marco Rubio, R-Fla., a leading member of the Gang of Eight, says he won't support "any law that does not ensure the enforcement things happen."
Still, the White House and Senate Majority Leader Harry Reid, D-Nev., are optimistic reform will happen.
"Well, it's certainly going to pass the Senate," Reid said Sunday in an interview with ABC's George Stephanopoulos. "It would be a bad day for our country and for the Republican Party if they continue standing in the way of this."