WASHINGTON (AP) — Plea bargain negotiations between criminals and prosecutors will now come under constitutional scrutiny because a divided Supreme Court ruled Wednesday that convictions can be overturned if defense lawyers don’t adequately assist clients in deciding whether to accept such offers.

The court’s decision could affect nearly every criminal case in the United States, where more than 9 in 10 convictions come by guilty pleas.

In a rare move justices use to underscore their objections, Justice Antonin Scalia read his dissent aloud from the bench. He said the court’s decision “upends decades of our cases … and opens a whole new boutique of constitutional jurisprudence” — plea bargaining law — even though there is no legal right to be offered a plea bargain.

“In the United States, we have plea bargaining a plenty, but until today, it has been regarded as a necessary evil,” said Scalia, who was on the losing side of two 5-4 decisions on the issue. “…Today, however, the Supreme Court elevates plea bargaining from a necessary evil to a constitutional entitlement. It is no longer a somewhat embarrassing adjunct to our criminal justice system; rather as the court announces … ‘it is the criminal justice system.'”

The two majority opinions, both written by Justice Anthony Kennedy, have potentially broad impact because 97 percent of federal convictions and 94 percent of state convictions in 2009 were obtained by a guilty plea, according to the Justice Department.

The rulings crafted by Kennedy mean that criminal defense lawyers are now required to inform their clients of plea bargain offers, regardless of whether they think the client should accept them, and must give their clients good advice on whether to accept a plea bargain at all stages of prosecution. If they don’t, Kennedy said, they will run afoul of the Sixth Amendment guarantee that criminal defendants have a right to assistance of counsel.

“The right to counsel is the right to effective assistance of counsel,” Kennedy said.

In one case decided by the court, Galin Edward Frye was never told by his lawyer about plea bargain offers from Missouri prosecutors before he pleaded guilty, before trial, to driving with a revoked license. In the second case, Anthony Cooper rejected a plea offer after getting incorrect legal advice from his lawyer, and then was convicted by a jury in Michigan of assault with intent to murder and other charges and sentenced.

In both cases, Kennedy said the defense lawyers’ actions violated their client’s constitutional rights. The rulings sent the cases back to lower courts.

“This court now holds that, as a general rule, defense counsel has the duty to communicate formal offers from the prosecution to accept a plea on terms and conditions that may be favorable to the accused,” Kennedy said. “… When the defense counsel allowed the offer to expire without advising the defendant or allowing him to consider it, defense counsel did not render the effective assistance the Constitution requires.”

In the second case, “if a plea bargain has been offered, a defendant has the right to effective assistance of counsel in considering whether to accept it,” Kennedy said. “If that right is denied, prejudice can be shown if loss of the plea opportunity led to a trial resulting in a conviction on more serious charges or the imposition of a more severe sentence.”

Kennedy, often the swing vote between the court’s liberal and conservative justices, was joined in both opinions by the liberals, Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

Scalia, however, said plea bargaining is not “covered by the Sixth Amendment, which is concerned not with the fairness of bargaining but with the fairness of conviction.”

Lawmakers, not judges, should solve the problem, “for example, by penalizing the attorneys who made such grievous errors,” Scalia said, noting that such lesser, non-constitutional remedies aren’t available to courts.

In Frye’s case, a lower court will still have to decide whether prosecutors would have been required to stick to their offer to Frye, because he was rearrested on the same charge of driving with a revoked license less than a week before his preliminary hearing.

Frye’s August 2007 arrest was his fourth on the same charge, so he was charged with a felony with a maximum sentence of four years. Missouri prosecutors originally offered Frye two deals, including pleading to a misdemeanor with a sentence recommendation of three months. With no knowledge of that offer, Frye pleaded guilty and got three years in prison. The Missouri Court of Appeals overturned his conviction.

Cooper was arrested after shooting a woman in the buttocks, hip and abdomen in 2003. He was charged with assault with intent to murder and three other charges, but prosecutors offered to drop two charges and recommend a maximum of seven years and one month in prison if he pleaded guilty to the others. Cooper’s lawyer told him to reject the deal, saying incorrectly that prosecutors couldn’t prove murder because he had shot the woman below the waist. Cooper rejected the deal and was sentenced to a maximum of 30 years in prison. The 6th U.S. Circuit Court of Appeals in Cincinnati overturned Cooper’s conviction.

Kennedy said Michigan prosecutors should offer Cooper the same plea bargain as before and if he accepts it, the original state trial judge could then void all his convictions and accept the deal, vacate some convictions and resentence Cooper or leave his convictions and original 15-to-30 year sentence unchanged.

That remedy concerned Justice Samuel Alito, a former U.S. attorney in New Jersey.

“In my view, requiring the prosecution to renew an old plea offer would represent an abuse of discretion in at least two circumstances: first, when important new information about a defendant’s culpability comes to light after the offer was rejected, and second, when the rejection of the plea offer results in a substantial expenditure of scarce prosecutorial or judicial resources,” said Alito in a separate dissent in that case.

Scalia was joined at least partly in both dissents by Chief Justice John Roberts and Justice Clarence Thomas. Alito joined Scalia’s dissent in the Frye case.

“In today’s cases, the court’s zeal to bring perfection to everything requires the reversal of perfectly valid, eminently just convictions,” Scalia said. “It is not wise; it is not right.”

Scalia, who has mocked other justices for considering foreign law in making decisions, noted that many countries don’t allow plea bargaining in serious cases.

“In Europe, many countries adhere to what they aptly call the ‘legality principle’ by requiring prosecutors to charge all prosecutable offenses, which is typically incompatible with the practice of charge-bargaining,” Scalia said. “Such a system reflects an admirable belief that the law is the law, and those who break it should pay the penalty provided. ”

The cases are Missouri v. Frye, 10-444 and Lafler v. Cooper, 10-209.