The Dodd-Frank Wall Street Reform and Consumer Protection Act -- all 848 pages of it -- was widely recognized as one of the most sweeping piece of legislation ever enacted against the industry.

But, quite strikingly, Congress did not change the rules regarding so-called secondary liability for securities fraud -- rules that allow the Securities and Exchange Commission, but not the plaintiffs' bar, to bring cases against companies and individuals who allegedly aid or assist the commission of a fraud. Instead, Congress commissioned a study to examine the potential effects of such legislation before deciding whether to enact it. That was a wise and careful course, one that made sense to almost everyone except, of course, the plaintiff's bar.

Having failed to seek immediate legislative change, the plaintiff's bar and its amici returned to the Supreme Court in December. They had been there twice before, but, undaunted, they tried yet again. In Janus Capital Group Inc. v. First Derivative Traders, they replayed their same tired arguments and asked the court to provide what Congress did not: a private cause of action against alleged aiders and abettors. Hopefully, the third time is not a charm.

The Supreme Court has long held that one antifraud provision of federal securities law -- Section 10(b) of the Securities Exchange Act of 1934 -- applies only to "primary" actors, such as companies that issue misleading prospectuses, and not to "secondary" actors, such as the lawyers, accountants and other advisers that assist public companies in preparing these prospectuses. Under Section 10(b), parties may bring damages suits for securities fraud only against primary actors, not secondary actors operating behind the scenes.

For more than a decade, the SEC and class-action plaintiffs' bar have lobbied Congress for a statutory amendment to extend Section 10(b). Congress has chosen to allow the SEC to pursue secondary actors but -- despite repeated legislative proposals by Sen. Arlen Specter -- has withheld comparable authority from private plaintiffs. In light of this history, the Supreme Court has squarely concluded that "further expansion" of the private action "is for Congress," not the courts.

Congress considered the issue in the Dodd-Frank Act. Concluding that expanded liability proponents had not adequately made their case, Congress required the comptroller general to conduct a study on the impact of authorizing private plaintiffs to bring securities fraud actions against secondary actors.

There are good reasons to be worried about expanding Section 10(b) to include secondary actors. The Supreme Court itself has emphasized that "secondary liability ... exacts costs that may disserve the goals of fair dealing and efficiency in the securities markets." Service providers to public companies, such as the lawyers who participate in drafting prospectuses and the accountants who review financial statements, may be forced to increase their fees because of the risk that they will be sued as secondary actors. These increased costs would then be incurred by the companies' investors.

Extending secondary liability also risks undermining the predictability required by the securities markets, at a time when the markets already display unprecedented volatility. Any test for secondary liability is inherently malleable and amorphous, asking whether someone who made no false statement of his own nonetheless provided enough aid or assistance to someone else to warrant the imposition of liability. Given the opportunity, the plaintiffs' bar would undoubtedly exploit such uncertainty by bringing swarms of strike suits, the main purpose of which would be to force settlements from defendants wishing to avoid the enormous costs of litigation and the risk of multimillion-dollar judgments.

It is therefore no surprise that the plaintiffs in Janus urged the court to impose liability on a secondary actor. The plaintiffs alleged that the Janus family of mutual funds issued misleading prospectuses to the market. But they did not sue the Janus Funds; instead, they sued a separate company (and its corporate parent) that provides investment advisory services by contract with Janus. The federal court of appeals considering the case held that the investment adviser could be liable for "helping" the funds prepare their prospectuses -- a classic form of secondary liability. The Supreme Court granted review.

Tellingly, in their briefs, the plaintiffs did not even mention the Dodd-Frank Act. The omission was no accident. The Dodd-Frank Act represents Congress' considered judgment not to permit (at least not yet) the very type of lawsuit that the plaintiffs' bar wants the courts to entertain. The Supreme Court should not disagree and permit that which Congress already denied.

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Jonathan Cohn is a partner in the Washington office of Sidley Austin LLP and formerly served as a deputy assistant attorney general in the U.S. Department of Justice.