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.