Defense lawyers say there are 191 reasons serial killer Gary Lee Sampson should not be executed. With Sampson’s death penalty trial nearing an end, federal prosecutors say that’s too many.

Sampson’s lawyers have proposed listing the unprecedented number of mitigating factors for jurors, who may begin deciding as soon as this week whether Sampson gets the death penalty.

Closing arguments are slated for Wednesday.

A mitigating factor is a way of explaining a defendant’s actions to help guide a jury away from a death sentence. Jurors are asked to consider mitigating factors listed on a verdict slip. They also consider aggravating factors that argue for the death penalty.

Federal prosecutors have argued throughout the trial that defense attorneys have gone too far in proposing 191 mitigating factors, unfairly tilting the deliberation process in the defense’s favor.

The 191 mitigating factors proposed by the defense in a court filing earlier this month were scaled back from an original proposal last year of 308 factors, an unprecedented number.

According to Federal Death Penalty Resource Counsel, which tracks federal capital punishment cases, the majority of cases list anywhere from 20 to 40 factors.

Jurors who sentenced Boston Marathon bomber Dzhokhar Tsarnaev to death last year considered 21 factors. In Sampson’s original sentencing trial in 2003, jurors reviewed 17 factors.

Sampson was sentenced to death in 2003 for the carjacking and killing of Jonathan Rizzo, 19, and Philip McCloskey, 69, during a violent spree over several days in 2001. He also killed Robert “Eli’’ Whitney, 58, in New Hampshire, that same week.

A judge vacated the jury’s 2003 sentencing decision after finding wrongdoing by one of the jurors, and prosecutors have sought capital punishment again.

Sampson remains convicted of his crimes. Under federal law, the sentencing trials determine only whether he gets the death penalty or life in prison without parole.

Since Sampson’s second sentencing trial began in September, defense lawyers have sought to portray Sampson as an emotionally troubled, mentally ill man with learning disabilities who suffered years of abuse, including abuse in several prisons before his 2001 crimes.

They argued that he had an emotional breakdown in the summer of 2001 before carrying out the murders, and tried to turn himself in to the FBI.

A phone call to that agency was dropped by a dispatcher, and Sampson went on and committed his crimes. Defense attorneys argued that jurors should be able to consider all those factors.

Prosecutors have argued in court records that some of the proposed factors “have nothing to do with Gary Lee Sampson’s background, record, character or the circumstances of Sampson’s offenses, and thus are not relevant to the individualized sentencing decision the jury must make regarding Sampson.’’

In a court filing last Thursday, the prosecutors argued that Sampson lists multiple factors that argue the same point: For instance, his lawyers listed 30 factors related to the trauma and harsh conditions he suffered in prisons. Prosecutors argued that the point could be made in one factor.

Seven of the proposed mitigating factors relate to Sampson’s call to the FBI. Prosecutors argued the point could be made in one. And Sampson proposed 18 factors to make the point that he suffered a troubled childhood.

“Sampson’s December 2016 list is, by and large, an unwieldy list of facts presented at the current hearing, several of which are not particularized to Sampson, himself,’’ prosecutors argued, adding that “Sampson continues to attempt to confuse the jury by impermissibly skewing the weighing process in his favor.’’

Milton J. Valencia can be reached at milton.valencia-@globe.com. Follow him on Twitter @miltonvalencia.