On Tuesday morning the U.S. Supreme Court heard oral arguments in what has been widely described as both the most important business case of the term and the most important securities case of the decade.

Though everyone advises against making predictions based on justices’ questions at oral arguments, everyone does so anyway. For what it’s worth, it seemed to me the Court was tending in the pro-business direction by about a 5-3 margin. (Justice Stephen Breyer did not participate.)

Chief Justice John Roberts stressed that in recent years Congress has been active in defining precisely which sorts of securities fraud actions private parties – as opposed to the Securities and Exchange Commission – should be allowed to bring, and that he felt the plaintiffs were asking the Court to “expand” upon those remedies. “We did that sort of thing in 1971,” the Chief Justice said, “But we haven’t now for some time.”

The case, known as Stoneridge Investment Partners v. Scientific-Atlanta, will determine how easy or difficult it will be for plaintiffs lawyers bringing class actions for alleged securities fraud to sue third-parties in addition to the corporation that issued the stock in question. The third parties most frequently targeted are accounting firms, law firms, investment banks, and vendors who did business with the issuer.

Today, if third parties formally sign documents that are included in an issuer’s SEC filings — the way auditors do when companies file their annual reports — there is no question that they can be held liable as “primary violators” if they make false statements. But if their role is anything less direct than that, they can currently be sued, if at all, only under a controversial theory known as “scheme liability”: I.e., they are accused of having committed acts with the “purpose and effect” of furthering the issuer’s allegedly fraudulent scheme. In the Stoneridge case, the Court will either accept or reject “scheme liability” as a legitimate basis for suing third parties in private class-action suits. (The Court is not expected to rule until several weeks after the oral argument, at a minimum.)

In today’s arguments, Chief Justice Roberts and Justices Anthony Kennedy, Samuel Alito, and Antonin Scalia all asked tough questions of Stanley Grossman of Pomerantz Haudek Block Grossman & Gross, who was representing plaintiff Stoneridge, while generally letting Stephen Shapiro of Mayer Brown Rowe & Maw, who argued for the defendants, off more easily. If Justice Clarence Thomas (who, as is his custom, asked no questions) votes with the other conservatives, that would make a majority. (Deputy Solicitor General Thomas Hungar also argued for the United States, supporting the defendant.) On the steps of the courthouse afterwards, former SEC commissioner Joseph Grundfest, the co-director of Stanford University’s Rock Center on Corporate Governance, said – after stressing the usual provisos about the futility and inadvisability of making such predictions – that his count was at least 5-3, and possibly 8-0. (Grundfest had joined a friend-of-the-court brief opposing the concept of scheme liability.)At a stand-up interview being televised elsewhere on the plaza, Nina Totenberg told Stoneridge’s counsel Grossman that, after watching the argument, she couldn’t see how he could get five votes. Grossman replied, “I wasn’t counting.”

The recurring theme of the conservative justices’ questions was that they could not see a practical difference between “scheme liability” and the older, more familiar theory known as “aiding and abetting” liability, which the U.S. Supreme Court barred private plaintiffs from invoking in securities fraud cases in the 1994 case known as Central Bank v. First Interstate Bank. Even Justice David Souter asked plaintiffs counsel Grossman at one point, “are you making a distinction that in the real world is not a distinction?”

On the other hand, questions from Justices Ruth Bader Ginsburg, John Paul Stevens, and, at times, Souter, too, suggested that they might still see room for a meaningful distinction to be drawn between the two concepts.

Six of the nine justices on the Central Bank court are still sitting. Three were in the majority that disallowed the aiding and abetting theory in that case – Justices Kennedy, Scalia, and Thomas – while three were among the dissenters: Justices Ginsburg, Souter, and Stevens. Justice Kennedy, who is often now seen as the Court’s swing vote – because he is the most moderate member of the five-justice conservative faction – wrote the pro-business majority opinion in Central Bank.

More than 30 interested outside groups have submitted “friend-of-the-court” briefs. Briefs in support of the scheme liability concept have been submitted by several of the nation’s largest pension funds, attorneys general for more than 30 states, major labor unions, AARP, the Consumers Federation of America, and the trial lawyers trade group, now known as the American Association for Justice. Briefs opposing the concept have been filed by the U.S. Chamber of Commerce, the major securities exchanges, the securities industry, accounting industry, banking industry, insurance industry, law firms, and law firm insurers. The Solicitor General of the United States has weighed in on the side of the business community – i.e., opposing the scheme liability concept – though it did so over the objections of the Securities and Exchange Commission, which voted, 3-2, to support the concept. (All key briefs are available here.)

Stoneridge specifically focuses on a fraud committed in 2000 by officials of cable operator Charter Communications (CHTR). (Several Charter officials ultimately pled guilty to criminal charges in connection with these acts.) To inflate its revenue, Charter asked two of its set-top box vendors, Scientific-Atlanta (a unit of Cisco (CSCO)) and Motorola (MOT), to bill it $17 million more than previously agreed upon, and then to use that extra money to buy advertising from Charter, which Charter then improperly booked as revenue. The vendors allegedly assisted in the scheme by backdating contracts and providing phony invoices and correspondence to help Charter deceive its accountants into approving the bogus revenue recognition. Neither vendor misrepresented its own finances to its own shareholders, and Charter’s shareholders never directly saw any of the misleading documents prepared by the vendors.

In April 2006 the U.S. Court of Appeals for the Eighth Circuit (in St. Louis) rejected the scheme liability concept and dismissed Stoneridge’s case against the vendors. Two months later, the U.S. Court of Appeals for the Ninth Circuit (in San Francisco) came out the other way in a case known as Simpson v. AOL Time Warner, approving the scheme liability concept. (Time Warner (TWX) is the parent of Fortune‘s publisher.)

The most famous scheme liability case is one that is not directly before the Court, but whose presence will obviously loom large at the argument. After the Enron catastrophe, class-action impresario Bill Lerach filed a scheme liability case on behalf of holders of Enron securities against more than ten banks who had allegedly engaged in dubious transactions with Enron whose only apparent purpose was to help Enron draw up misleading financial statements. Lerach has already recovered $7.3 billion in settlements in the case from such banks as Citibank (C), J.P. Morgan Chase (JPM), and CIBC (CM). But in March 2007, the U.S. Court of Appeals for the Fifth Circuit (in Houston) ruled the same way the Eighth Circuit had, rejecting “scheme liability” and tossing out the case against the banks who had not yet settled, which included Merrill Lynch (MER), Credit Suisse (CS), and Barclays (BCS). (Lerach himself is scheduled to plead guilty on October 29 to conspiring to obstruct justice by bribing plaintiffs and deceiving judges in more than 150 shareholder class actions over more than two decades.)

The Stoneridge case is in one respect a very difficult case to decide but, in another, perhaps, quite easy. The difficult part is that, however the court rules, it will make a decision that works some real injustice to someone. If it rejects scheme liability, investors who have been hurt by mammoth frauds that have led to corporate bankruptcies will be unable to seek reimbursement from deep-pocketed third-parties who really do bear some responsibility for what happened to them. On the other hand, if the Court endorses the scheme liability theory, innocent third-parties will routinely and inevitably be joined as defendants in scores of frivolous cases and — having no way to get those cases dismissed at an early, inexpensive stage (i.e., on a “motion to dismiss”) — will be induced to pay extortionate settlement payments.

The potentially easy part of the case is that it may have already been effectively decided 13 years ago. “Scheme liability” sounds an awful lot like “aiding and abetting liability” — i.e., the notion that plaintiff shareholders should be able to sue third-parties who aided and abetted the issuer’s fraud. The U.S. Supreme Court rejected that theory, however, in its 1994 ruling in Central Bank v. First Interstate Bank. Though that 5-4 ruling was controversial at the time, Congress subsequently made its peace with that ruling — twice! In 1995, it restored by statute the right of federal prosecutors and the Securities and Exchange Commission to prosecute and sue aiders and abettors, but it specifically chose not to restore that right to private plaintiffs. Simply put, Congress decided that private securities actions were so subject to abuse that the costs to society of allowing private parties to sue alleged aiders and abettors were just not worth the benefits.

Congress then made exactly the same cost-benefit determination in 2002, when it passed the Sarbanes-Oxley legislation. Again it was asked to restore the right of private plaintiffs to sue aiders and abettors of securities fraud, and again it said no. Instead, it expanded the SEC’s power to impose fines and disgorgements on aiders and abettors – i.e., forcing them to cough up their profits – and then empowered the SEC to distribute those sums to defrauded investors in partial reimbursement for their losses. (These sums are not, however, as much as private plaintiffs could recover; so far the SEC in the Enron case has recovered about $400 million for shareholders, compared to the $7.3 billion collected by Lerach.)

Obviously, the plaintiffs in Stoneridge (and Enron) claim that scheme liability is distinguishable from aiding and abetting liability; they claim that with scheme liability the third-party has to be shown to have played a slightly more active role in the fraud than had been required to establish aiding and abetting liability, although the precise definition of that magic extra oomph has varied depending on the court and the facts of the case. The defendants and their amici argue, on the other hand, that scheme liability is just a semantic ploy; it’s old wine in new bottles. For what it’s worth, to me scheme liability and aiding and abetting liability sound like one and the same thing.

In an earlier post on these issues, see here, I came down on the liberal side of this dispute, because of the unfairness of depriving shareholders of the right to sue parties who aided and abetted in the frauds that injured them. (There are very few other areas of the law where aiders and abettors are not liable to the same degree as principals.) But after reading many of the briefs from both sides in Stoneridge, I’ve changed my mind. These issues were decided in 1994, and Congress has twice consciously chosen not to overrule the part of that Court decision that barred private suits against aiders and abettors, which is what Scientific-Atlanta and Motorola really were (if anything) here. Congress decided — reasonably — that shareholder litigation is so fraught with abuse, and is such a grotesquely inefficient and ineffective way of reimbursing fraud victims, that it was wiser to leave the deterrence and punishment of aiders and abettors to the SEC and federal prosecutors.