Democratic and Republican senators are in tense discussions over a border-security amendment to the Gang of Eight immigration-reform package that could prove critical to its passage, Sen. John McCain, R-Ariz., said Tuesday afternoon.

Whether this proposal can satisfy Republicans’ security concerns without losing Democrats should be clear by Wednesday, McCain said. The amendment, a project of Sens. Bob Corker, R-Tenn., and John Hoeven, R-N.D., would alter the Gang of Eight bill by creating a specific border security strategy that the administration would have to implement and spell out what metrics to use to determine when the border is secured.