If all this is is a matter of nomenclature - political science is not a science because most of its practitioners are non-scientists - then any argument is absurd, for one could just concede the point, call a certain subset of political scientists 'scientific political scientists' and exclude the nonscientific practitioners, and that would be the end of it.
I think it is beyond a matter of nomenclature, it is a matter of method. As you said, science is a method. Branches of human knowledge and pursuit that follow this method are generally agreed to be "scientific." For example, biology is considered a scientific medium because the scientific method is necessary for any meaningful insight into it. To continue my previous example, the scientific method is NOT a necessary requirement to get meaningful insight into sports ... however, it is possible to apply the scientific method in that analysis. The question is whether "political science," as a field, is one that necessitates the use of the scientific method: if so, then it can be considered a "true science" (not that this really means anything, to be honest ... but this is the subject raised by the OP).
The meaningful question here is not, "Are there enough scientific Political Scientists to call Political Science a science?"
It is, "Under what circumstances, if any, might it be possible and useful to apply the scientific method to political systems; and is the goal of Political Science - the prediction and/or explanation of political events from generalized principles - attainable?"
For the purposes of addressing the OP, as in whether or not "political science" is a "true science" (once again, I actually think this kind of thing is pretty arbitrary and pointless, but for the sake of the argument I want to try to keep things in the frame of what the OP means to address), I think a better question is whether or not the goals of political science necessarily require the application of the scientific method to attain any useful insight. I don't think there is any question as to whether or not it is possible in some circumstance to think "scientifically" about a field or a subject ... but if we are trying, as the OP is, to draw a hard line in the sand and determine whether or not political science is, indeed, a science, I think we need to determine the role of the scientific method in attaining whatever goals.
"Spare me your space age technobabble, Atilla the Hun." -Zap Brannagan
"It means they could've done some freaky human/demon hybrid thing." -Nightstalker, on Nazis