A senior lawmaker proposed a controversial piece of legislation on Wednesday that directs NASA to pick a second company to build the agency’s next Moon landers — in addition to Elon Musk’s SpaceX, which was awarded a $2.9 billion NASA contract to build a lander earlier this year. The bill hasn’t passed the full Senate yet, but it marks a new front in an ongoing effort to overturn or rejig NASA’s decision. It also sets up the first political challenge for NASA’s new administrator, former Sen. Bill Nelson.

NASA’s choice of SpaceX last month to build the agency’s first lunar lander since 1972 spawned a wave of opposition from some lawmakers and the two losing companies in the running: Jeff Bezos’ space firm Blue Origin and Dynetics. Those companies lodged formal protests against NASA’s decision, triggering a procedural pause on SpaceX’s new contract. Among other things, the protests maintain that NASA should have picked two firms instead of one.

Amid a lobbying effort from Blue Origin, those calls have found their way into a NASA authorization bill, proposed as an amendment to the Endless Frontier Act by Sen. Maria Cantwell (D-WA), chair of the Senate Commerce Committee overseeing NASA. Cantwell represents Blue Origin’s home state of Washington. Under Cantwell’s language, NASA would be required to reopen the competition within 30 days and allow it to use $10 billion of its budget to pick a second lunar lander provider.

Before choosing SpaceX, NASA had been expected to pick two companies, a strategy that guaranteed a backup in case one lander fell behind. But the agency went only with SpaceX — its bid was half of Blue Origin’s — after funding shortfalls from Congress. “It was in NASA’s best interest, along with the budget that was there, for us to award to one,” NASA’s human spaceflight chief, Kathy Lueders, who led the decision to pick SpaceX, said last month.

Simply adding another company to build NASA’s Moon lander, as proposed in Cantwell’s amendment, could run afoul of the existing agreements with SpaceX, agency officials say. “It’s not as simple as picking the next in line,” says one person familiar with the process, speaking anonymously to chat frankly about legal matters. Going through the long and protracted process of picking another company would also endanger NASA’s rush to get to the Moon by 2024, agency staff say. (Blue Origin argues the opposite: that not picking a second company risks the 2024 goal.)

NASA declined to comment on the bill, citing the ongoing litigation from Blue Origin and Dynetics’ protests.

Blue Origin argues NASA can simply add another company, and it’s well within the agency’s ability to do so legally. That’s because, company employees say, SpaceX’s award falls under a kind of research and development category of government contracting that would permit another player to join, unlike more legally rigid contract programs for routine transportation services.

It’s unclear what specific legal jargon could (or could not) permit NASA to pick another company to develop a lunar lander alongside SpaceX. But Cantwell’s proposed amendment aims to assert a straightforward solution.

SpaceX’s $2.9 billion award is for two flights to the Moon using Starship, the company’s fully reusable rocket system that’s still under development in Texas. The first mission would require Starship to make an uncrewed lunar landing, followed by another landing carrying astronauts.