You can find the brief of Public Citizen, Texas RioGrande Legal Aid, NAACP’s Austin and Texas branches, NAACP-Legal Defense Fund, Lawyers’ Committee for Civil Rights Under Law, American Civil Liberties Union, and
People for the American Way here.
You can find the Brennan Center’s amicus brief here.
MALDEF filed a brief as well that I have not seen. Trvais County also filed a brief.
If anyone has links to the government’s brief or to any other briefs, please send them along. You can find extensive coverage and analysis of the constitutional issues from a number of election law scholats at this link.
UPDATE: The plaintiffs’ motion for summary judgment is here.