June 12, 2013 -- Yesterday, the immigration bill took a big step forward when it cleared a procedural hurdle in the Senate. But today showed how difficult the path to passage could be.

Lawmakers on Wednesday afternoon sparred on the Senate floor over an amendment that would essentially rewrite the border security language of the Gang of Eight bill. Top Republicans have said that the proposal is necessary to win over enough of their colleagues to secure passage for the legislation. But Democrats have dubbed the measure a "poison pill" that places a roadblock on the path to citizenship for undocumented immigrants.

The proposal has been dubbed "the Cornyn amendment" for its sponsor, Sen. John Cornyn (R-Texas).

You can read all 134 pages here. But in brief, it creates tougher "triggers" that need to be met for formerly undocumented immigrants to obtain permanent legal status. The current bill also contains "triggers" for citizenship to take effect, but Democrats worry Cornyn's could be too difficult to achieve.

Here's a short summary of the requirements that will need to be met before people would be able to legalize under Cornyn's plan:

* "Full situational awareness" (translation: continuous monitoring) of the entire U.S.-Mexico border.

* "Operational control," meaning that 90 percent of all illegal border crossers are apprehended or turned back in every southern border sector in a given year.

* All U.S. employers must use the E-Verify system to check the immigration status of job applicants.

* A biometric entry/exit system in place at all air, land and sea ports to the U.S.. The system is designed to prevent visa overstays.

The biggest contrast to the underlying bill is that the Cornyn amendment requires that the federal government meet all these benchmarks before legalized immigrants can obtain permanent residency.

"My amendment would guarantee such a mechanism and it would guarantee the results that Washington has long promised but never delivered," Cornyn said in a statement.

But his amendment means that if federal officials have determined the border-security measures have not been yet, these immigrants could be stuck with tenuous, temporary status for far longer than the 10 years laid out in the Gang of Eight bill. Under the current legislation's "trigger," the border plan must be be implemented, but not completely achieved before legalized immigrants can get green cards.

One of the authors of the immigration bill, Sen. Chuck Schumer (D-N.Y.), said on the floor that he would be open to other border security changes from Republicans. But he said that Cornyn's triggers "would make a path to citizenship … possibly yes possibly no."

"That is unacceptable," he said.

Gang of Eight members have also dubbed the Cornyn plan too costly. Schumer said that a universal biometric system like Cornyn's could cost up to $25 billion. A similar proposal was rejected during committee hearings and, instead, a more limited fingerprinting system at airports was adopted.

On the floor, Sen. John McCain (R-Ariz.) chastised Cornyn for claiming that his amendment, which also includes 5,000 additional border patrol agents, would not add to the cost of the bill.

"There's a finite amount of money," authorized in the bill, McCain said. "[It's] simple, first-grade mathematics."

But Republicans who support immigration reform still face the task of winning more of their colleagues' votes. The Cornyn amendment has been presented by leaders as the measure that could assuage the fears of the bill's critics.

Senate Minority Leader Mitch McConnell (R-Ky.) on Tuesday called it "the key amendment" to winning the trust of most Republicans. Sen. Jeff Flake (R-Ariz.), a Gang of Eight member, said that Cornyn is sincere in his willingness to back the bill if his amendment is adopted.

"I can tell you, he wants to strengthen the bill," he said at a breakfast sponsored by the Christian Science Monitor. "And he has said himself he would like to vote for the bill."

But immigrant-rights advocates aren't so sure. Here's what Frank Sharry, executive director of America's Voice, told Fusion last week.

"He pretends to be sincere about the need for reform," he said. "He asks for changes that are a bridge too far. He destabilizes the bipartisan agreement already in place. He helps to thwart reform."

This post was updated at 6:31 PM