In an open letter, the union representing US Citizenship and Immigration Services (USCIS) employees said the measure "will damage public safety and national security and should be opposed by lawmakers."
A Senate committee recently began consideration of the vast immigration reform bill backed by President Barack Obama, which would lead to legal status for millions of people in the country without papers.
The process could last weeks before moving to a second stage on the Senate floor, where Democrats hold a slender majority. The Republican-controlled House of Representatives would then take up the legislation. Its prospects, however, remain uncertain.
Under Obama's presidency, "the culture at USCIS encourages all applications to be approved, discouraging proper investigation into red flags and discouraging the denial of any applications," the union alleged. "USCIS has been turned into an 'approval machine.'"
The union also said it was not consulted by the so-called bipartisan Gang of Eight senators who drafted the bill, echoing an argument made by another union representing US Immigration and Customs Enforcement (ICE) workers.
"Instead, the legislation was written with special interests -- producing a bill that makes the current system worse, not better," it said.