Daniel Medwed, a professor at Northeastern University School of Law who opposes the death penalty, said he believes it is unlikely the defense will sway the judge in this case. But he said they are likely throwing down a marker now, preserving this as potential fodder for later appeal.

“This is a long-term play,” Mr. Medwed said.

The defense is also mounting an argument that Mr. Tsarnaev’s hospital-bed statements to law enforcement agents shortly after his capture should be suppressed. Investigators initially questioned Mr. Tsarnaev before reading him his Miranda rights under a public-safety exception.

But his lawyers claimed his statements were involuntary and that his due-process rights were violated. His injuries included a gunshot wound to the head that likely caused traumatic brain injury, the defense attorneys wrote. They also said he was on several pain drugs that impaired his judgment, had to write responses in a notebook and repeatedly asked for both rest and a lawyer.