By now, every member of Congress should have received a very long letter about the Green New Deal. Sent by 626 environmental groups on Thursday, the letter calls on lawmakers to support the idea, popularized by freshman Congresswoman Alexandria Ocasio-Cortez, of a sweeping economic stimulus package to fight global warming.

Thursday’s letter addresses one of the most common criticisms of the Green New Deal: that no one can agree about what should be in it. These 626 environmental groups, including Greenpeace, the Center for Biological Diversity, and 350, say a Green New Deal should include an expansion of the Clean Air Act; a ban on crude oil exports; an end to fossil fuel subsidies and fossil fuel leasing; and a phase-out of all gas-powered vehicles by 2040, among many other things.

But the letter also shows how far Ocasio-Cortez and her allies still have to go to in gathering support for the Green New Deal, because six of the largest, most influential environmental advocacy groups didn’t sign it: the Sierra Club, the Natural Resources Defense Council, the Environmental Defense Fund, Mom’s Clean Air Force, Environment America, and the Audubon Society. Two green groups founded by deep-pocketed Democratic celebrities are also absent: Al Gore’s Climate Reality Project and Tom Steyer’s NextGen America.

It’s not that they weren’t asked to sign. “There was a big effort to get signatures, and it’s great to see activists pushing for a Green New Deal with the urgency and scale climate change requires,” said a spokesperson for a national environmental group that didn’t sign the letter. “But the details matter... There is some language that gave us some concern.”

Representatives from other groups were also largely hesitant to explain on the record why they didn’t sign the letter. (Perhaps, like some Democrats, they’re afraid that Ocasio-Cortez might scold them on Twitter.) Speaking on background, though, some said the letter did not allow for enough flexibility on the details of a Green New Deal—such as one section promising that all signatories will “vigorously oppose” a deal that includes “market-based mechanisms and technology options such as carbon and emissions trading and offsets, carbon capture and storage, nuclear power, waste-to-energy and biomass energy.”