Safety and consumer advocates are urging senators to reject a parliamentary maneuver to secure passage for stalled legislation governing self-driving cars by attaching it to a must-pass reauthorization for the Federal Aviation Administration.

The AV START Act, which would create a framework for regulating deployment of automated vehicles on public roadways, has strong bipartisan support, but is being held up by several Democrats who say the bill in its current form gives industry too much leeway to ignore basic safety standards when the technology remains unproven. Absent those objections, the bill could pass by unanimous consent without a vote.

In a letter to senators, more than five dozen groups said the AV START Act deserves to be fully reviewed and debated, and have amendments, because it could guide auto safety policy for decades. The FAA bill is also an inappropriate legislative vehicle, they argued, because there is a stark difference in how federal aviation policy handles autonomous systems compared to the self-driving bill.

“The FAA has rigorous protocols for ensuring the safety of automation in the air, and examples of the success of effective standards and oversight of automated systems fly over our heads every single day,” the letter said. “Conversely, the AV START Act, in its current form, would shockingly allow potentially millions of vehicles on the market to be exempt from meeting existing safety standards.”

Self-driving cars hold great promise for reducing the more than 37,000 annual fatalities on America’s highways, but safety advocates are questioning many assumptions after a handful of deadly accidents involving Uber and Tesla vehicles. They want federal investigations into the accidents to be completed and improvements made to the legislation, which passed the Commerce Committee, before it moves ahead to the full Senate for a vote.

The FAA reauthorization has been stalled since last year too because of several controversial provisions, but the Commerce Committee is now accepting amendments and seeking floor time for a potential vote this summer, according to Politico. That prompted Advocates for Highway and Auto Safety and its allies, to send the letter, spokesman Eric Naing said.

Still, opportunities to pass the AV START Act are quickly dwindling as Congress faces other legislative priorities and few remaining weeks before adjourning for election season.