Federal lawmakers have been toying with for months now, but that effort is appearing to stall out, following two recent crashes involving an Uber-owned self-driving car and a Tesla Model X.

The proposed legislation would allow automakers to eventually annually that are exempt from existing auto safety standards, up from the current total of 2,500 vehicles. But the bill faces an “uncertain path,” reports The Detroit News, after drew more attention to the fledgling technology, especially from vocal critics.

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Safety advocates have urged lawmakers to pump the brakes on the measure, but Peters and other supporters say they are still trying to put the bill on President Donald Trump’s desk this year.

A similar measure has been passed by the U.S. House. Senate backers have been hoping to win unanimous consent for their version of the self-driving bill, but at least five senators have publicly expressed concerns about the measure.

A group of Democratic senators voiced concerns about the bill—called the AV START Act—even prior to the crashes, the News reports, saying they’re “concerned that the bill indefinitely pre-empts state and local safety regulations even if federal safety standards are never developed.”

One sticking point is a provision that would prohibit states from adopting regulations pertaining to autonomous vehicles design, construction, software or communication. But states would have the ability to regulate registration, licensing and traffic laws.

Still, the situation shows just how tenuous the support is for autonomous vehicles. And if crashes like this keep happening, it’s only going to get harder from here to generate enough support for workable legislation.