This is why I said that for more complicated aspects you can create a equation type question and write a more complicated equation. Here an example that you most probably can adapt easily. You can use nested ifs or you can use separated ifs (not tested, you will play around with this and see if it works 100%.
Code:
{if(A1==1 OR 5 OR 15, "left-wing", if(A1==2 OR 3 OR 13, "right-wing", "not defined"))}
or
Code:
{if(A1==1 OR 5 OR 15, "left-wing", "")}
{if(A1==2 OR 3 OR 13, "right-wing", "")}
{if(A1==4 OR 6 OR 7 OR 8 OR 9 OR 10 OR 11, "center", "")}
Let's assume this equation question is called "spectrum", you can call the result of this equation which would be either "right-wing", "left-wing" or "center" via {spectrum}
With so many different parties I would actually have each group (e.g. left wing) with consecutive numbers, because it makes your formula easier.
Let's assume the parties with answer code 1-5 are left wing, the parties 6-15 are right wing and the parties 16-19 are center. Then you could just write something like this:
Code:
{if(A1 <= 5, "left wing", if(A1 <= 15, "right wing", "center))}