In 1971, Lewis Powell, a well-connected corporate lawyer, wrote a confidential memo to an official at the U.S. Chamber of Commerce. “The American economic system is under broad attack,” Powell warned, by everyone from “New Leftists and other revolutionaries” to academics, the media, and politicians. It was time for the right, and the Chamber itself, to fight back. And among the tactics the Chamber should employ, Powell wrote, was an aggressive push to shift the federal courts in a pro-business direction.

Six years later, Powell’s vision became a reality, when the Chamber launched a new litigation arm that aimed to accomplish that shift, in large part by strategically filing amicus briefs in cases of interest to the business community. By that time, Powell himself sat on the Supreme Court.

On Thursday came the latest evidence of just how successful that effort has been. A new report by the Constitutional Accountability Center (CAC), a liberal judicial group, at which MSNBC got an exclusive first look, finds that since the start of the Roberts Court in 2005, the Chamber has won more than two-thirds of the cases in which it’s gotten involved. And this term, it has weighed in to support corporate interests in nearly a quarter of the cases heard by the nation’s highest court. Just as important, it’s been more active than any other organization in working to shape the court’s agenda by pushing it to take up favored cases.

“The Chamber is not just winning more cases, they’re participating in more cases,” Doug Kendall, CAC’s founder and president, told MSNBC. ”So the impact of their litigation is far beyond what it used to be.”

The major result of the Chamber’s success, legal scholars say, has  been a string of rulings that threaten to block the courthouse door to ordinary Americans looking to hold corporations accountable. And with court-watchers’ attention focused on higher-profile gay marriage and voting rights cases this term, it’s a development that’s flown largely under the radar.

The Roberts Court’s pro-business outlook has been apparent for several years. But the CAC report suggests it may be accelerating. Both the Chamber’s participation rate and its success rate have risen significantly in recent years. This term, the Chamber filed amicus briefs in 24% of cases, up from 10% during the latter part of the Rehnquist Court, from 1994 to 2005, a period of stability when there were no changes to court personnel. And since John Roberts became Chief Justice, the Chamber has won 69% of the cases in which it’s gotten involved (see chart below). That’s up from 56% during the latter part of the Rehnquist Court, and just 43% during the last five years of the Burger Court, from 1981 to 1986.

Of course, the Roberts Court would likely have had a pro-business tilt even if the Chamber didn’t exist. But the business lobby has also helped shape which cases the high court hears in the first place—an issue on which outside groups likely do affect the court’s decisions, though the effect is difficult to quantify. The Justices typically grant around 75 or 80 out of roughly 8,000 petitions submitted. And when they’re deciding which ones, amicus briefs from well-known players like the Chamber offer a “helpful signal” for the court, David L. Franklin, an associate professor at the DePaul College of Law, and the author of a 2009 study of the Chamber’s record at the Roberts Court, told MSNBC.

Between May 2009 and August 2012, the business lobby filed 54 amicus briefs at the cert. stage—that is, when the court is considering which cases to take up—far more than any other private organization, SCOTUSblog reported earlier this month. And it succeeded in convincing the Justices to take the case in nearly a third of those instances—a better success rate than that of any other group that filed more than ten briefs.

“They are very much shaping the diet of cases that goes before the Supreme Court each term,” said Kendall. “And then once they get a case before the court they usually win it.”

The Chamber didn’t respond to a request to discuss its success at the Supreme Court. But its litigation arm, the National Chamber Litigation Center (NCLC), boasts some heavy-hitting lawyers who know the court and its conservative justices well. Its top lawyer for regulatory litigation, Rachel Brand, served as an assistant attorney general at the Bush Justice Department, where she helped shepherd Justices Roberts and Samuel Alito through their Senate confirmation hearings, and previously was a law clerk to Justice Anthony Kennedy.

The upshot of the Chamber’s success has been a shift in the legal landscape that disadvantages employees and consumers taking on large corporations.

“The key themes in the cases that the Chamber has managed to get the court’s attention on have been making it harder for plaintiffs to prevail, and in some cases making it harder for plaintiffs to get into court,” Franklin said.

One example came last month, when the Justices ruled by 5-4 in favor of Comcast in a class-action lawsuit, citing what it said was a flaw in the class action’s proposed damages model (NBC Universal is owned by Comcast). The effect, said Franklin, is to “make it harder for plaintiffs to band together” to launch class action lawsuits, a key mechanism for groups of individuals to bring cases against corporations.

A 2011 ruling by the same 5-4  margin had a similar effect. The court’s conservative Justices found that AT&T couldn’t be subject to a class action suit after falsely claiming their wireless plan included a free cellphone. Because the phone had come with a fine-print contract containing an arbitration clause, the court ruled that the Federal Arbitration Act prevented the plaintiffs from banding together. Again, the upshot was to make it far harder for consumers to file class action lawsuits.

The Chamber submitted amicus briefs in support of both Comcast and AT&T.

“If you think you were defrauded by your credit card company or cellphone company or cable company, and you want to take that company to court and join together with others to do so, that has been rendered almost impossible” as a result of these and several other recent rulings, said Kendall. “So for consumers, it’s had a very dramatic impact on the way you can get justice in this country.”