One of the most ardent opponents of the comprehensive immigration reform bill that's now on the Senate floor is warning Republican lawmakers that the bill would endanger lives and “guarantee future illegal immigration.”

In a letter obtained by the International Business Times, Chris Crane, president of the National Immigration and Customs Enforcement Council, a union that represents about a third of the government's Immigration and Customs Enforcement, or ICE officers, seeks to move the bill further to the right or derail it altogether as Senate debate heats up. In his letter, Crane urges Republican Sens. John Cornyn of Texas and Marco Rubio of Florida to introduce an amendment that would give more authority to ICE agents to detain and deport undocumented immigrants. It would limit the Obama administration’s authority over ICE officers.

Crane has railed against comprehensive reform as “blanket amnesty” that will “provide a path to citizenship for the most criminal street gang,” and has emerged in recent months as a top adversary of the effort while lobbying the Senate “Gang of Eight” to include his demands in the bill. A significant presence on Capitol Hill these days, he has been the most frequent witness since this year’s immigration debate began, according to the New York Times. This is also not the only letter he has sent to the Senate in his drive to derail reform. 

Crane’s letter is likely to give ammunition to opponents of the bill, who have echoed his warnings in the past. "Our interior enforcement needs are almost totally neglected in the Gang's proposal," Sen. Jeff Sessions, R-Ala., said in April. "Alarmingly, the bill leaves intact the single greatest obstacle to immigration reform: the administration's abuse of prosecutorial discretion to prevent the enforcement of federal law."  

A thorn in the administration’s side, Crane has been clashing with top officials at ICE, which is housed in the Department of Homeland Security and is responsible for enforcing the nation's immigration laws, over President Barack Obama’s directive to use “prosecutorial discretion” to target undocumented immigrants who have committed crimes rather than those who are simply living here without authorization. In the letter, Crane said the administration prohibits officers from “arresting and removing … convicted criminals and aliens incarcerated in jails.” He wants some of Obama’s instructions reversed by the Senate bill. 

“Absent drastic improvements to the interior enforcement provisions, there is no doubt that S.744 will undermine public safety, officer safety, and constitutional rule of law, and that it will guarantee future illegal immigration,” Crane wrote in his most recent letter. “Clearly, we will not solve our nation’s immigration problems, or fix this bill without empowering ICE agents and restoring and improving ICE’s deteriorated enforcement capacity.”

In his letter, Crane urges the senators to incorporate security measures similar to bills put forward on the House side that increase resources for ICE, including controversial legislation by Rep. Trey Gowdy, R-S.C., that would give local law enforcement authority to enforce local immigration laws. Rubio, a member of the Gang of Eight that helped craft the legislation, has in recent days expressed support for increasing the bill’s border security provisions.

"Senator Rubio has met with Mr. Crane and other ICE officials, and we welcome their suggestions for how to improve the legislation,” Rubio spokesman Alex Conant told IBTimes. “We all agree that the current immigration system is badly broken, which is why we're working to secure our borders, strengthen interior enforcement, and modernize our legal immigration system."

Cornyn's office responded to a request for comment by noting that a border security amendment he proposed already boosts interior enforcement. Top Democrats have rejected the amendment as a “poison pill.”