When one political party succeeds in eliminating the opposition party from all political offices, does that then make the one party a dictatorship? Well, for one thing, it does eliminate any need for accountability, thus making it an inviting habitat for political wolves wearing the proverbial sheep's clothing. Also it makes closed door meetings no problem, followed by legislative initiatives to outlaw ballot referenda or recalls.
However, openness would eliminate any need for secret-combination politicians to have to meet in private to redraw legislative and congressional district voting maps. A democracy, with openness and with a participating elected opposition (as was intended by our founders) flourishes; without it, a one-party dictatorship takes control.