Sen. Jeff Flake, R-Ariz., confirmed Thursday that immigration negotiators have agreed on a compromise border security amendment that the Arizona Republican said should pave the way for the "Gang of Eight" package to clear the Senate's 60-vote threshold.

Flake, a "Gang of Eight" member, said key Democrats have signed off on the amendment offered by Sens. Bob Corker, R-Tenn., and John Hoeven, R-N.D. The amendment's final language was still being drafted and particular details finalized, but Flake said the proposal would mandate both border security strategy and new security infrastructure that the executive branch would be required to implement.

"We've made a push for 70 votes and we think it's [still] possible," Flake told reporters during a conference call.

The "Gang of Eight" comprehensive immigration reform package includes triggers to ensure that currently illegal immigrants could not become permanent residents, and ultimately U.S. citizens, unless the legislation's border security provisions are met. But several Republicans viewed those triggers as too weak, forcing reform proponents to consider amendments to strengthen them.

Flake said the Corker-Hoeven amendment would mandate -- among other measures -- a doubling of the current number of border patrol agents; the building of 700 miles of border fencing and that a nationwide e-verify program preventing employers from hiring illegal immigrants be completely up and running. This list of border security tasks is being referred to by negotiators as a "punch list." Flake believes that the fence in particular, could be the key to garnering GOP support.

However, the Arizonan conceded that Republicans dropped what had been a crucial demand -- that the trigger be bolstered with metrics for border security effectiveness. In other words, that the Department of Homeland Security not just implement a border security strategy designed by Congress, but that it must be demonstrably reducing illegal immigration.

Republicans had been pushing Democrats to adopt one metric that would have required that 90 percent of illegal immigrants who are "identified" had to be arrested or deported. "That is not in the bill," Flake said.

Corker and Hoeven are expected on the Senate floor Thursday afternoon to unveil their amendment.