WASHINGTON — Criminal defendants have a constitutional right to effective lawyers during plea negotiations, the Supreme Court ruled on Wednesday in a pair of 5-to-4 decisions that vastly expanded judges’ supervision of the criminal justice system.

The decisions mean that what used to be informal and unregulated deal making is now subject to new constraints when bad legal advice leads defendants to reject favorable plea offers.

“Criminal justice today is for the most part a system of pleas, not a system of trials,” Justice Anthony M. Kennedy wrote for the majority. “The right to adequate assistance of counsel cannot be defined or enforced without taking account of the central role plea bargaining takes in securing convictions and determining sentences.”

Justice Kennedy, who more often joins the court’s conservative wing in ideologically divided cases, was in this case in a coalition with the court’s four more liberal members. That alignment has sometimes arisen in recent years in cases that seemed to offend Justice Kennedy’s sense of fair play.

The consequences of the two decisions are hard to predict because, as Justice Antonin Scalia said in a pair of dissents he summarized from the bench, “the court leaves all of this to be worked out in further litigation, which you can be sure there will be plenty of.”

Claims of ineffective assistance at trial are commonplace even though trials take place under a judge’s watchful eye. Challenges to plea agreements based on misconduct by defense lawyers will presumably be common as well, given how many more convictions follow guilty pleas and the fluid nature of plea negotiations.

Justice Scalia wrote that expanding constitutional protections to that realm “opens a whole new boutique of constitutional jurisprudence,” calling it “plea-bargaining law.”

Scholars agreed about its significance.

“The Supreme Court’s decision in these two cases constitute the single greatest revolution in the criminal justice process since Gideon v. Wainwright provided indigents the right to counsel,” said Wesley M. Oliver, a law professor at Widener University, referring to the landmark 1963 decision.

In the context of trials, the Supreme Court has long established that defendants were entitled to new trials if they could show that incompetent work by their lawyers probably affected the outcome. The Supreme Court has also required lawyers to offer competent advice in urging defendants to give up their right to a trial by accepting a guilty plea. Those cases hinged on the right to a fair trial guaranteed by the Sixth Amendment.

The cases decided Wednesday answered a harder question: What is to be done in cases in which a lawyer’s incompetence caused the client to reject a favorable plea bargain?

Justice Kennedy, joined by Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan, acknowledged that allowing the possibility of do-overs in cases involving foregone pleas followed by convictions presented all sorts of knotty problems. But he said the realities of American criminal justice required to the court to take action.

Some 97 percent of convictions in federal courts were the result of guilty pleas. In 2006, the last year for which data was available, the corresponding percentage in state courts was 94.

“In today’s criminal justice system,” Justice Kennedy wrote, “the negotiation of a plea bargain, rather than the unfolding of a trial, is almost always the critical point for a defendant.”

Quoting from law review articles, Justice Kennedy wrote that plea bargaining “is not some adjunct to the criminal justice system; it is the criminal justice system.” He added that “longer sentences exist on the books largely for bargaining purposes.”

One of the cases, Missouri v. Frye, No. 10-444, involved Galin E. Frye, who was charged with driving without a license in 2007. A prosecutor offered to let him plead guilty in exchange for a 90-day sentence.

But Mr. Frye’s lawyer at the time, Michael Coles, failed to tell his client of the offer. After it expired, Mr. Frye pleaded guilty without a plea bargain, and a judge sentenced him to three years.

A state appeals court reversed his conviction but said it did not have the power to order the state to reduce the charges against him. That left Mr. Frye roughly where he started, with the options of going to trial or pleading guilty without the benefit of a plea deal.

Justice Kennedy wrote that Mr. Frye should have been allowed to try to prove that he would have accepted the original offer. But that was only the beginning of what Mr. Frye would have to show to get relief. He would also have to demonstrate, Justice Kennedy wrote, that prosecutors would not have later withdrawn the offer had he accepted it, as they were allowed to do under state law. Finally, Justice Kennedy went on, Mr. Frye would have to show that the court would have accepted the agreement.

There was reason for doubt that Mr. Frye could prove that prosecutors and the court would have ended up going along with the original 90-day offer, as Mr. Frye was again arrested for driving without a license before the original plea agreement would have become final.

Justice Scalia, in a dissent joined by Chief Justice John G. Roberts Jr. and Justices Clarence Thomas and Samuel A. Alito Jr., called all of this “a process of retrospective crystal-ball gazing posing as legal analysis.”

The second case, Lafler v. Cooper, No. 10-209, concerned Anthony Cooper, who shot a woman in Detroit in 2003 and then received bad legal advice. Because all four of his bullets had struck the victim below her waist, his lawyer incorrectly said, Mr. Cooper could not be convicted of assault with intent to murder.

Based on that advice, Mr. Cooper rejected a plea bargain that called for a sentence of four to seven years. He was convicted, and is serving 15 to 30 years.

Justice Kennedy rejected the argument that a fair trial was all Mr. Cooper was entitled to.

“The favorable sentence that eluded the defendant in the criminal proceeding appears to be the sentence he or others in his position would have received in the ordinary course, absent the failings of counsel,” he wrote.

A federal judge in Mr. Cooper’s case tried to roll back the clock, requiring officials to provide him with the initial deal or release him. Justice Kennedy said the correct remedy was to require the plea deal to be re-offered and then to allow the trial court to resentence Mr. Cooper as it sees fit if he accepts it.

Justice Scalia, here joined by Chief Justice Roberts and Justice Thomas, said this was “a remedy unheard of in American jurisprudence.”

“I suspect that the court’s squeamishness in fashioning a remedy, and the incoherence of what it comes up with, is attributable to its realization, deep down, that there is no real constitutional violation here anyway,” Justice Scalia wrote. “The defendant has been fairly tried, lawfully convicted and properly sentenced, and any ‘remedy’ provided for this will do nothing but undo the just results of a fair adversarial process.”

Stephanos Bibas, a law professor at the University of Pennsylvania and an authority on plea bargaining, said the decisions were a great step forward. But he acknowledged that it may give rise to gamesmanship.

“It is going to be tricky,” he said, “and there are going to be a lot of defendants who say after they’re convicted that they really would have taken the plea.”

Justice Kennedy suggested several “measures to help ensure against late, frivolous or fabricated claims.” Among them were requiring that plea offers be in writing or made in open court.