While it is certainly ethically and morally questionable - do consider how much you are willing to take this battle as a representative of justice or students or whatever.
You can avoid conflicting your own principles, if you write a heading/preamble,
Please note, this is an assignment essay (prof Y, course X, University Z) and does NOT reflect my personal standpoint regarding the legislation (H.R. 592 / S. 109). This is the assignment of X.Y, the essay will be signed as "Donald Duck" to avoid any issues. (your signature here)
And then write a flaming support for the legislation. Sign with Donald Duck. Sign your own name in the preamble.
The professor has already made a serious error. He is liable to take a lot of flak - if the right story comes along. If this recieves a failing grade, or anything less than an A+, then go public - and say you recieved unfair grading due to having a political view that differs from the professor. This is a story that is easily sold, and will ride the wave of offendedness that seems to be on the rise these days.
He really cannot grade you punitively, or something that can be percieved as punitively - he must give you a better grade than what you deserve.
Update: You ask for advice what to do next. It seems it was a mistake on your professors part. Assume no malice. Let it be.