Lawmakers want to revise key elements, such as letting millions of illegal immigrants stay in the US, favouring skills and education over families and setting out the terms of a new temporary worker programme.

Any one of the changes has the potential to sink the whole measure, which was unveiled with fanfare Thursday but was still being drafted late Friday.

Commerce Secretary Carlos Gutierrez, who helped negotiate the compromise, called it "very well-balanced," and cautioned against revisions that could upset the framework.

"You take something out and you're creating a problem throughout the system you may think that you're only tweaking one part," Gutierrez said in an interview. "We've got to be very careful as to what is proposed to change."