With his bow tie, unfailingly polite questions and calm demeanor, Justice John Paul Stevens appears to be an unlikely revolutionary. But in the past decade, he led an effort that overhauled key elements of the criminal justice system.

It began in 2000 with his majority opinion in Apprendi v. New Jersey, holding that due process required that any fact increasing the penalty for a crime above the prescribed statutory maximum must be proved to the jury beyond a reasonable doubt. And it culminated with U.S. v. Booker in 2005 in which Stevens dismantled the mandatory character of federal sentencing guidelines. In the process, he put together an unusual coalition, finding key support from justices Antonin Scalia and Clarence Thomas.

An overriding concern for due process — fairness — in the criminal justice system was the driving force behind his views in Apprendi. It also animated Stevens’ rulings in two other keys areas of criminal law that will be major parts of his legacy — the death penalty and the right to counsel.

“I think for Justice Stevens his sentencing opinions are part and parcel of his due process principles,” said sentencing scholar Douglas Berman of Ohio State University Michael E. Moritz College of Law. “For him, this has always been about defendants’ rights, not the formality of jury trials. If you read Apprendi, you get the sense that, in order to get Thomas and Scalia to come along, he threw the Sixth Amendment into the mix.

“That leads me to believe part of making this revolution happen was not just writing the opinion in Apprendi but understanding he wouldn’t get five votes just on due process. It was the jury trial right that Scalia waxed about, and that would be the hook for the broader principle,” Berman said. “That was Stevens’ genius.”

The seeds of this sentencing shift were sown much earlier by Stevens, according to some criminal law scholars. Berman traces it to Stevens’ important vote in the Gregg cases — a trio of decisions in 1976 reinstating the death penalty in Georgia, Texas and Florida.

“He is among the majority suggesting that if we add some law to capital sentencing, we may get to a point where it is done well enough,” Berman said. “They in some sense presage the efforts he’s made to bring more law to noncapital sentences.”

James Liebman of Columbia Law School and Lawrence Marshall of Stan­ford Law School, both former Stevens clerks, have described the justice’s approach to the death penalty as “less is better.” In Thompson v. Oklahoma (1988) and Atkins v. Virginia (2002), he wrote majority opinions striking down capital punishment for those below age 15 and for mentally retarded persons. He also is credited with being particularly influential in Roper v. Simmons (2005), written by Justice Anthony Kennedy, eliminating the death penalty for persons under 18.

“In Baze v. Rees, he makes the full Blackmun-esque journey,” said criminal justice scholar Christopher Smith of Michigan State University, referring to Stevens’ 2008 concurrence in that lethal-injection challenge. Stevens concludes that the death penalty “with such negligible returns to the state” is unconstitutional. Justices Harry Blackmun and Powell, both supportive like Stevens of the death penalty in 1976, ultimately changed their view.

Stevens often held criminal defense lawyers to a higher standard of competency than the Court’s conservative majority in recent years, but he succeeded in moving the law on the right to counsel in the current term with his majority opinion in Padilla v. Kentucky. He held that defense counsel has an affirmative duty to inform a client that a plea may carry a risk of deportation.

“Much of what he has done is be a voice and conscience as opposed to defining what the law will be for the nation in these areas,” Smith said, “and I don’t see anyone else playing that role.”

Marcia Coyle can be contacted at mcoyle@alm.com.