WASHINGTON (Reuters) - A landmark U.S. bill that would speed adoption of self-driving cars is now “a long shot” to pass before the current Congress adjourns in the coming weeks, a key Republican senator behind the legislation said.

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Many automaker lobbyists and congressional aides say the bill would face even tougher odds in 2019 when Democrats and Republicans will share control of Congress.

Some Democrats and safety advocates say the bill does not do enough to ensure the safety of self-driving cars and threatens the ability of states to oversee autonomous vehicles. Supporters of autonomous vehicles, who say they can save lives, believe the bill is needed to speed their adoption and overcome regulatory barriers that were written for human-driven vehicles.

“It’s a long shot but we have successfully knocked down a lot of the barriers,” Senator John Thune, who chairs the Commerce Committee, said in an interview in his Capitol Hill office late Tuesday. “It seems like every time we clear one they put another one up.”

Thune and other senators are trying to clear objections from some remaining Democratic senators. One issue that has emerged in recent days is whether automakers would agree to a five- to seven-year sunset clause that would require Congress to revisit self-driving car laws in the 2020s.

Staff for Thune and Democratic Senator Gary Peters earlier this month circulated a draft of a revised bill aimed at breaking a legislative stalemate. Peters said in a statement Wednesday he was “focused on finding areas of agreement with our colleagues and getting a bill signed into law before we adjourn at the end of the year.”

Thune said negotiations have been taking place in recent days with staff for a handful of Democratic senators who have raised some safety concerns. “If we can demonstrate that we have the votes to pass something here, we can get the House on board,” Thune said.

He added that while the measure was still “alive. it’s not real alive.”

The pair have been working for more than a year to try to win approval of the bill by the Senate.

Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers, a group representing major automakers said Wednesday, “Without legislation from Congress, lots of important work that would benefit Americans slows down... We will keep pushing for legislation to pass.”

The Republican-led U.S. House unanimously approved a measure in September 2017, but it has been stalled in the Senate for over a year. A Senate bill would allow automakers to each sell up to 80,000 self-driving vehicles annually within three years if they could demonstrate they are as safe as current vehicles.

General Motors Co in January filed a petition with U.S. regulators seeking an exemption to use vehicles without steering wheels as part of a ride-sharing fleet it plans to deploy in 2019 but has receive no decision.

Alphabet Inc’s Waymo unit launched a limited commercial autonomous ride-hailing service in Arizona earlier this month.