With U.S. markets fretting Tuesday at the prospect of a downgrade in the government’s triple-A credit rating, you may be wondering: Who can we sue? Litigation, after all, is practically an unalienable American right. The problem, however, is that any attempt to sue the credit rating agencies for downgrading U.S. securities will run smack into the Bill of Rights. The rating agencies, as many a disgruntled mortgage-backed securities investor has discovered in the last few years, are shielded from liability because their ratings are considered to be public opinion protected by the First Amendment of the U.S. Constitution.