The main site for the cartoonist-led effort in support of a recent push by a group of mayors to serious curtail gun violence can be found here. Ruben Bolling, who spearheaded the effort, talks about doing so here.
That's an interesting group of cartoonists involved, about as ecumenical a bunch in terms of format and approaches as you're likely to find working together on anything. I also sort of like that this is the first I'm hearing of it, that Bolling decided to go with this group rather than subject the effort to the widest possible campaign to make sure everyone that wanted to be involved at this initial stage got to be involved. That would be a very comics-culture approach, and I believe not as effective as simply roping together a small bunch of talented cartoonists and putting something out there.
Anyway, I'm always uncertain as to why more cartoonists aren't involved politically in this kind of public advocacy way, although I guess alienating audience members can be a concern, particularly for certain models of revenue. That's also always fascinating to me, that people would feel alienated from someone and their work for a political view they don't necessarily share, but that's largely true of the whole of modern American life now.