- US Constitution, Article 1 Section IX Clause 3
Undoing the mergers sounds like ex post facto law. I'd appreciate it, Senator Warren, if you would amend the constitution first. And I can't wish you luck with that amendment.
You can certainly pass laws to undo a merger or (equivalently) break up a company.
But you can create a law that Amazon (and potentially others) will be in violation of, and following proper trial, they are broken up.
Also, just to reinforce the point other folks have mentioned - that clause doesn't work like that.