One of those groups is largest pension fund in the country, the California Public Employees Retirement System, which filed a lawsuit in state court in California in July, claiming that “wildly inaccurate ratings” had led to roughly $1 billion in losses.

Image Richard Cordray, Ohio’s attorney general, sued the nation’s largest credit rating agencies, alleging that they cost the state’s pension funds $457 million by approving securities that went bust. Credit... Kiichiro Sato/Associated Press

And more litigation is likely. As part of a broader financial reform, Congress is considering provisions that make it easier for plaintiffs to sue rating agencies. And the Ohio attorney general’s action raises the possibility of similar filings from other states. California’s attorney general, Jerry Brown, said in September that his office was investigating the rating agencies, with an eye toward determining “how these agencies could get it so wrong and whether they violated California law in the process.”

As a group, the attorneys general have proved formidable opponents, most notably in the landmark litigation and multibillion-dollar settlement against tobacco makers in 1998.

To date, however, the rating agencies are undefeated in court, and aside from one modest settlement in a case 10 years ago, no one has forced them to hand over any money. Moody’s, S.& P. and Fitch have successfully argued that their ratings are essentially opinions about the future, and therefore subject to First Amendment protections identical to those of journalists.

But that was before billions of dollars in triple-A rated bonds went bad in the financial crisis that started last year, and before Congress extracted a number of internal e-mail messages from the companies, suggesting that employees were aware they were giving their blessing to bonds that were all but doomed. In one of those messages, an S.& P. analyst said that a deal “could be structured by cows and we’d rate it.”

Recent cases, like the suit filed Friday, are founded on the premise that the companies were aware that investments they said were sturdy were dangerously unsafe. And if analysts knew that they were overstating the quality of the products they rated, and did so because it was a path to profits, the ratings could forfeit First Amendment protections, legal experts say.

“If they hold themselves out to the marketplace as objective when in fact they are influenced by the fees they are receiving, then they are perpetrating a falsehood on the marketplace,” said Rodney A. Smolla, dean of the Washington and Lee University School of Law. “The First Amendment doesn’t extend to the deliberate manipulation of financial markets.”