Monsanto has not threatened to sue any state over GMO food labeling. This is a myth first propagated by the Organic Consumers Association (OCA) in Vermont during the 2012 Vermont legislative season.
Monsanto responded in a blog post, correcting the record and pointing out that the Vermont Attorney General and Vermont Legislative Council, in fact, raised legal constitutionality questions in an Agriculture Committee hearing.
The source of the claim is the Organic Consumers Association. They said it. We never did.