The trial's final phase starts at 10:30 a.m. (1:30 p.m. ET) Thursday. It's unclear how long it will take.
One thing is clear, Toobin said.
"This is the last decision the jury has to make," he said, "and things have not been going well for (Arias) so far."
There are 127 people on death row in Arizona. If Arias is given a sentence of death, she would be the fourth woman on death row in the state.
Minutes after the first-degree murder guilty verdict was announced last week, Arias said receiving a sentence of life in prison without parole would be the worst possible outcome.
"I said years ago that I'd rather get death than life, and that still is true today," she told Phoenix television station KSAZ. "I believe death is the ultimate freedom, so I'd rather just have my freedom as soon as I can get it."