Updated

The union representing thousands of Immigration and Customs Enforcement employees is calling on Republican Sen. Marco Rubio to leave the tight-knit group negotiating an immigration overhaul.

Rubio is considered the most politically vital member of the so-called "Gang of Eight," since his support could give conservatives cover to ultimately vote for legislation Congress for years has struggled to pass. Rubio is one of four Republicans in the group, which is preparing to introduce legislation in a matter of days.

But National ICE Council President Chris Crane issued a lengthy statement Friday saying the union is not confident the bill would address the flow of illegal immigration. He specifically raised concern that, based on the assessment of one Democratic member, the plan could legalize millions of illegal immigrants before enforcing border security.

"I would then respectfully call on Senator Rubio to follow through on his commitment to the American people -- and his pledge to accomplish enforcement before legalization -- and to leave the Gang of 8," Crane said in a statement.

The proposal, though, is said to ratchet up border surveillance and make other security changes. Rubio spokesman Alex Conant, in response to the ICE union statement, said the forthcoming proposal is "the start of the process."

"Our hope is that rather than attacking something they haven't seen, people will give good-faith consideration to the proposal after it's introduced and suggest ways to improve it," he said. "The border security and enforcement measures in our proposal will result in the toughest immigration and border enforcement in U.S. history. There will be tough but achievable hurdles that tie border security directly to the pathway to citizenship, and if these triggers are not part of the legislation, there won't be a bipartisan deal."

Rubio has so far defended the Gang of Eight, while vowing to push for an open process once the bill is introduced. He has demanded that border enforcement be embedded in the language of the bill -- though there are lingering questions over how closely that would be tied to a plan to put illegal immigrants on a path to potential citizenship.

The ICE union also complains that the senators and the White House have not included them in talks on the bill, though unions and other advocacy groups have been involved.