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.

"We're in very intense negotiations," McCain told reporters. "The goal is to try to get us altogether to agree that border security is vital and that we need to do things that would satisfy everybody's concerns without losing Democrat support."

The existing Gang of Eight legislation delegates to the Department of Homeland Security the task of designing a border security strategy, and then allows DHS, an executive branch agency, to determine on its own whether the border is secure. This troubles Republicans, who do not trust either the current administration or future presidents to enforce the border or follow through on whatever security provisions are mandated in the law.

Hoeven said the proposed amendment is intended to be a bridge between Republicans who want to support overhauling U.S. immigration law, but aren't satisfied with the border security triggers in the current reform bill. The triggers are intended to ensure that the border is secured before 11 million newly legalized immigrants can become permanent residents and ultimately, U.S. citizens.

Hoeven confirmed that his amendment is also designed to win the support of Democrats who have oppose stronger border-security standards, like one offered by Senate Minority Whip John Cornyn, R-Texas. Cornyn's proposal would delay legalization, also referred to as Registered Provisional Immigrant status, until certain border security goals are attained, a nonstarter for Democrats.

"We're trying to find something that ensures we secure the border and something that can get broad, bipartisan support," Hoeven said. "I think that's important not only for our vote for the Senate, but for what happens in the House."

Hoeven said he's "hopeful" about his amendment's prospects. Here's how he described the proposal:

"We put the [border security] plan in the bill, No. 1. They can enhance it and add to it, but we put a plan — a strategic plan that they've got to attain. And, they can do more; they can't do less, that's important. Second ... they've got to have some verifiable standards, metrics, measurements that people can agree on and it won't be just DHS. They've got to have [other government agencies] working with them to make the transition from RPI to" permanent residency.