The future of net neutrality is in jeopardy again, so advocates are getting back to the fight. In a letter sent today to FCC chairman Ajit Pai, as well as Senators John Thune and Bill Nelson, over 170 groups ask Congress and the Federal Communications Commission to continue to support and protect the net neutrality rules put into place in 2015.

“[We] urge you and your colleagues to oppose legislation and regulatory actions that would threaten net neutrality and roll back the important protections put in place by the FCC in 2015 and to continue to enforce the Open Internet Order as it stands,” the groups write.

The letter comes just a day before the Senate Commerce Committee’s first oversight hearing of the FCC while Pai is in charge. Thune is chairman of the committee, and Nelson is ranking member, which is why they’re included on the letter. Tomorrow will be the committee’s first chance to grill Pai on the direction he’s taking the commission — the future of the FCC’s privacy rules is likely to come up.

Coincidentally enough, the letter was also sent just hours before Pai was nominated by President Trump for another five-year term at the commission.

Signatories of the letter include the American Civil Liberties Union, the American Library Association, the Electronic Frontier Foundation, Greenpeace USA, and the Writers Guild of America, West, among many others. (Also included: The Harry Potter Alliance, an activist group of Harry Potter fans.)

Pai has said he believes using Title II to enact net neutrality was “a mistake,” and Thune believes the issue should be resolved in Congress. It’s far from clear how this is going to play out, but either way, opponents of Title II are now in control of net neutrality’s future, and these groups know it’s going to be a long fight to keep those protections intact.