I certainly wouldn't like that they took such an action, even if I liked whatever candidate they were stanning for. It would come off as pretty classless to most people I think.

But should it be illegal? IMO -- no. If this is the hill that some company wants to die on, let them try. Why not?

Thought experiment: If there was a political candidate running on a platform to destroy the internet, I think it would be perfectly reasonable for internet companies to vouch for the competition.