There are multiple grounds. A popular approach so far has been to argue the proper process for rule changes hasn't been followed. I'm not sure that applies her

There are multiple grounds. A popular approach so far has been to argue the proper process for rule changes hasn't been followed. I'm not sure that applies here, but in many cases there are legal steps to changing regulation. You can't just wake up Tuesday and say "I think stop signs are going to be green." Separately and more applicable in this case, states can argue they're being harmed by this particular set of federal rules. This can further be tied to already-existing law, essentially arguing that a new regulation is incompatible with a law passed by Congress. As an example, the EPA can't simply approve dumping industrial waste chemicals into waterways, because the Clean Water Act says otherwise.