Here’s what I have so far (in addition to the Former Republican FEC Commissioners’ brief already linked):
BCRA Sponsors (McCain, Feingold, Shays, and Meehan)
Brennan Center
Campaign Legal Center, etc.
Institute for Justice
League of Women Voters (and see this blog post)
Reporters’ Committee for Freedom of the Press
Scholars Brief (Hayward, Dimino, Jones, LaRaja, Miylo, Munger, New, Primo, Samples) (more Hayward on CU here)
UPDATE: More briefs:
Center for Competitive Politics
American Independent Business Alliance (Demos)
California Broadcasters Association, etc.
Justice at Stake (on the effect of overruling Austin on judicial elections—as with the BCRA sponsors’ brief, perhaps calculated to sway Justice Kennedy)
Brief of 26 States (This brief goes heavy on the constitutional avoidance argument, opening with NAMUDNO on this point.)
I won’t link to any more of these briefs as the FEC has set up this page linking to even more supplemental amicus briefs.
All supplemental amicus briefs on both sides of the case are due today. Supplemental party reply briefs are due August 19.
The BCRA sponsors brief is going to be especially important, not only because it comes from the sponsors of the legislation, but also because Seth Waxman, their attorney, is likely to get 10 minutes to argue in favor of the law, and Seth, a former SG, is highly respected on the Court.
This brief is somewhat understated on the Austin distortion point, though not to the extreme extent of the government’s brief. The BCRA sponsors’ brief does not mention “distortion” or “corruption,” but does make arguments against corporate dominance of the electoral process. (The brief does so in a much more subtle way than the brief filed by the Campaign Legal Center, Democracy 21, etc., who had jointly worked on the initial amicus brief filed by the BCRA sponsors in this case.) But most of the BCRA sponsors’ brief is dedicated to the question of stare decisis. And, in a point that makes me very happy, the brief cites Ashwander and makes a brief constitutional avoidance argument. Still, it is striking that the two most important briefs supporting Austin do their best to keep away from Austin equality arguments—just a testament to the writer knowing his/her audience.