Feinstein, whose state is a leading research center for autonomous vehicle technology, has concerns about the overall safety of self-driving cars and about whether the technology is ready to be placed on public roadways, Commerce Committee Chairman John Thune, R-S.D., said.
Sen. Richard Blumenthal, D-Conn., has expressed concern about the need to require a fallback mechanism for a driver to take control of a vehicle in case of software failure, while Sen. Edward Markey, D-Mass., is concerned about data privacy and cybersecurity as vehicles evolve into networked devices.
Markey "continues to work with the leadership to strengthen provisions in the bill related to automotive defects, cyberattacks and consumer privacy," his communications director, Giselle Barry, wrote in an email.
Without those holds, Senate rules would let the legislation be adopted by unanimous consent, without the procedural hurdles of a regular Senate floor vote.
But one hold is enough to foreclose that option.
Thune, lead sponsor of the bill, said he hopes to accommodate the issues raised by the three Democrats "as long as it doesn't move it too far into a more regulatory, sort of heavy-government direction."
Ultimately, he and the other sponsors will have to decide whether to keep seeking a unified front, or face the gauntlet of a Senate floor vote.
"A lot of testing is going on in [Sen. Feinstein's] state, so I'm hoping folks will eventually be able to prevail on her to realize that this is eventually going to make roads safer, not less so," Thune told reporters after a field hearing held in conjunction with the Washington Auto Show.
Thune said he has talked with Majority Leader Mitch McConnell about scheduling floor time for a vote or hitching autonomous vehicle legislation to, for example, a potential infrastructure bill.
A stand-alone vote could require up to a week of Senate time to go through procedural steps, but the Senate's priorities in the next month include passing government funding and an immigration bill.
"There's lots of support for this bill," Thune said. "We listened a lot to stakeholders, worked with trial lawyers on liability issues, and clarified state and federal roles. I think we've got it to a point where it's a good, balanced bill. So I don't know why we shouldn't be able to get this across the finish line."
Unanimous passage of AV legislation would send a message to the public that Congress can work together on an issue poised to change society, said Eric Kennedy, who heads the automotive industry practice at the Buchalter law firm.
"This is a bit of a scary experience for folks to have a driverless car on the roads," he said. "People want to feel that their legislators get it, and if they can get a unanimous bill, the public will feel better."
For now, though, surveys show public skepticism about autonomous cars remains high.
A poll released this month by Advocates for Highway Safety found that 75 percent of the public is not comfortable with disabling equipment such as steering wheels and pedals in a computer-controlled vehicle.
The Senate bill's full name is the American Vision for Safer Transportation through Advancement of Revolutionary Technologies Act. It bars states from imposing restrictions on autonomous vehicle safety performance and development, allows automakers to win tens of thousands of exemptions from safety rules that require human controls, and sets requirements for disclosing how data is used.
Automakers and tech companies will be required to regularly report how they are working to meet safety standards, but there are no specific mandates on how to do so.
The auto industry supports the light regulatory touch, saying it is necessary to encourage investment in technology, while public-interest groups say the industry is rushing the technology to market without sufficient regard to public safety.