I did a little search and found the following list for "major
distributions" which looked somewhat unbiased:
Ubuntu
Mandriva Linux
SUSE LINUX
Fedora Core
Debian GNU/Linux
Knoppix
MEPIS Linux
Gentoo Linux
now knoppix is a derivitave of debian, so getting into debian testing
gets you into knoppix.
ubuntu is also a derivitave of debian, but they seem to be forking, so
may need to be addressed seperately.
I'd probably add red hat to the list too. (even though i've never
used it)
One possibility would be to focus the "campaigning" to cover these
distributions, plus a "support whatever distribution you personally
use" campaign. :)