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. http://www.organicconsumers.org/articles/article_25180.cfm(April 4, 2012)
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. http://monsantoblog.com/2012/04/06/we-said-what-in-vermont/.
The source of the claim is the Organic Consumers Association. They said it. We never did.