I like Robin's recent take on the question.

Donations to change policy within the partisan subspace, however, only achieve good when they happen to be on the right side of partisan disagreements. Averaged over the disagreeing parties, such donations cannot on average achieve good unless there is a correlation between between donations, or donation effectiveness, and which sides are right. Even if you think you are right at the moment on your particular partisan policy opinions, you can’t think it good on average to encourage partisan donations, unless you think donations tend overall to go to the good or more donation-effective sides.

For every person that sincerely believes that the Flurb Party will change things for the better and donates $100 to it, there's someone who believes that the Bleeg Party will change things for the better and donates $100 to it ... so they cancel each other out. Allocating a bigger part of the economy to printing fliers and posters doesn't seem like the best way to make the world a better place.