I believe on active duty you just can't be an officer of that particular organization. I would assume that the same rules apply while in ROTC. Also, don't wear any military related clothing or accessories as it would be inappropriate in such a setting. I hope this helps.
You can absolutely join political organizations on campus and in the community. You can serve as an officer. You may not wear your uniform, utilize your rank/position, or attempt to speak for ROTC, the military, or the government.