WASHINGTON, March 21 (Reuters) - The U.S. Supreme Court on Wednesday extended the constitutional right to effective legal assistance in cases of plea bargain deals that are rejected or lapsed due to bad lawyer advice.

Splitting by a 5-4 vote, with moderate conservative Justice Anthony Kennedy joining the court's four liberals in two cases, the majority held that the right to effective counsel applied to informal plea bargain negotiations that take place between criminal defendants and prosecutors.

In the second ruling, the majority held a defendant must show "a reasonable probability" the plea offer would have been accepted by the judge when a lawyer's bad advice caused the defendant to reject the plea bargain and then stand trial.

The pair of rulings in cases from Missouri and Michigan could allow convicted criminals to seek to reopen their cases after they passed up favorable plea bargains because of ineffective assistance of their lawyers.

In both cases, the criminals said they had been denied their constitutional right to effective assistance of counsel because of mistakes by their attorneys during plea negotiations. The majority agreed.

The rulings could have broad impact as about 95 percent of all criminal cases nationwide are resolved through a plea agreement, not a trial.

Kennedy in one of the opinions said that a defense counsel as a general rule has a duty to communicate to the client formal prosecution offers of a plea deal on terms and conditions that may be favorable to the accused.

To show harm when a plea offer has lapsed or been rejected because of a lawyer's bad performance, Kennedy said defendants must show they probably would have accepted the more favorable plea offer if they had received effective legal advice and that the plea deal would have been accepted in court.

When a defendant shows ineffective assistance has caused the rejection of a plea leading to more severe sentence at trial, the remedy must "neutralize the taint" of a constitutional violation," he said in the other opinion.

Chief Justice John Roberts and Justices Samuel Alito, Clarence Thomas and Antonin Scalia dissented.

COULD SOW CONFUSION

Scalia, who took the rare step of reading parts of his dissent from the bench, said there was no doubt that the defendants in the cases were convicted and sentenced under fair, constitutionally valid procedures.

"Until today, no one has thought that there is a constitutional right to a plea bargain," he said. "Today's opinion opens a whole new field of constitutionalized criminal procedure: the field of plea-bargaining law. The court announces this new field in opinions that almost seem designed to sow confusion."

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

In the Michigan case, Anthony Cooper passed up a plea deal that would have sent him to prison for four to seven years for shooting a woman four times.

His lawyer told him he would not be convicted of assault with intent to murder because he shot the woman below the waist and missed a shot to her head.

Cooper went to trial, was convicted of assault with intent to murder and on other charges, and received a sentence of up to 30 years in prison.

In the other case from Missouri, Galin Frye was charged with driving without a license in 2007. A prosecutor offered two alternative plea agreements.

In one, the prosecutor said Frye could plead guilty in exchange for a 90-day sentence. But Frye's lawyer at the time failed to tell him about the offers.

After the offers expired and after Frye was arrested again for driving with a revoked license, he pleaded guilty and received a sentence of three years in prison.

The Supreme Court's majority opinions sided with Frye and Cooper and sent the cases back to lower courts for further proceedings.

The Obama administration had supported the argument by prosecutors in the two cases that the constitutional right to counsel only ensured a fair trial, and that defendants do not have a right to a plea bargain, including one that results in a lighter sentence.

The Supreme Court cases are Lafler v. Cooper, No. 10-209, and Missouri v. Frye, No. 10-444.

For Lafler: John Bursch and B. Eric Restuccia, Michican Solicitors General.

For Cooper: Valerie Newman, Assistant Defender, State Appellate Defender Office.

For Missouri: Shaun Mackelprang, Missouri Assistant Attorney General.

For Frye: Emmett Queener, Craig Johnston and Melinda Pendergraph of Woodrail Centre.

(Reporting by James Vicini)

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