TUCSON, Ariz. » Judy Clarke, the public defender for the man charged in the Tucson shooting rampage, Jared L. Loughner, has made motions on his behalf and entered a plea for him of not guilty. But one of her most essential acts of lawyering came when she patted Loughner on the back in court last month, leaned in close and whispered in his ear.

For the small cadre of lawyers specializing in federal death penalty cases, getting the defendant to trust them, or just to grudgingly accept them, can be half the battle. That is especially true when mental illness is a factor, as it may be in the case of Loughner, a troubled young man accused of opening fire on a crowd on Jan. 8 in an attempt to kill Rep. Gabrielle Giffords.

In her unassuming, almost motherly way, Clarke excels at getting close to people implicated in awful crimes. In jailhouse meetings that can stretch most of the day, she listens intently and grows to know her outcast clients in a way few ever have in their troubled lives, colleagues say.

Still, Clarke, who has made a name for herself representing notorious murderers and terrorists, sometimes falls short. One client threatened to kill her during his trial. More than one has tried to dump her midway through.

How Loughner and Clark get along, or fail to, will set the course for how the criminal case unfolds. One of Clarke’s biggest challenges may be persuading Loughner to allow her to raise questions about his mental health; that issue led to conflict between Clarke and some of her previous clients, like the Unabomber, Theodore J. Kaczynski, and the al-Qaida operative Zacarias Moussaoui.

"It could go many different ways," said Michael First, a psychiatrist who has worked on a case with Clarke. "He could be totally acknowledging he’s mentally ill, or he could be the Kaczynski and Moussaoui type and be absolutely adamant there is nothing wrong with him."

Kaczynski severed ties with Clarke and the rest of his legal team when they pushed the idea of presenting his mental illness to the jury as a reason to spare his life. Once a mathematician, he was proud of his mind and found his lawyers’ suggestion offensive.

And Moussaoui, who faced the death penalty on charges that he helped plan the Sept. 11 attacks, opposed the efforts of his legal team, which Clarke was assisting, to portray him as mentally ill. First recalled spending hours outside Moussaoui’s cell, being rebuffed in his efforts to coax him into a conversation.

Next to nothing is known of what Loughner and Clarke have spoken about in the month since he was arrested. But it is unlikely, former colleagues of Clarke say, that she and her two co-counsels, Mark Fleming and Reuben C. Cahn, are very far along in planning his defense. It is possible, lawyers say, that they have not even broached the extent of Loughner’s mental illness or the shooting that left six dead and 13 wounded, among them Giffords, who is recovering in a rehabilitation center in Houston.

Clarke, rather than focusing on her clients’ innocence, spends much of her defense work trying to persuade jurors to spare her clients’ lives. She does this by presenting what lawyers call a "mitigating social history" — a narrative of abuse, violence or mental illness that the defendant may have suffered. She sends investigators to find grade-school teachers, former girlfriends, classmates, anyone who can provide insight into what made her client go awry.

Clarke rarely gives interviews to the news media, but she did explain her philosophy last year in a law school publication at Washington and Lee University in Virginia. "None of us, including those accused of crime, wants to be defined by the worst moment or worst day of our lives," she said.

In representing Susan Smith, a South Carolina mother who killed her two boys, Clarke focused the jury’s attention on the facts that Smith’s father had committed suicide and that her stepfather had sexually abused her.

"She was able to change her from Susan the monster to Susan the victim," said Tommy Pope, a South Carolina legislator who prosecuted the case against Smith. A jury spared her life.

In her court arguments, Clarke can be quite vehement, lawyers who have seen her at work say. Clarke once told The Los Angeles Times: "I like the antagonism. I like the adversarial nature of the business. I love all of that."

But her demeanor changes to that of a social worker when meeting with her clients one on one.

"Even people who are quite mentally ill can identify someone who is real and who wants to protect them," said David Bruck, a lawyer at Washington and Lee University’s School of Law who has worked with Clarke. "She’s a great listener, and she’s focused on the client. She tries to understand the client. The client becomes her world."