In the case of Commonwealth v. Joshua Lewis, the Massachusetts Supreme Judicial Court recently emphasized the limitations prosecutors are required to abide by when arguing their cases before juries.
In that case, the defendant was charged with assault with intent to murder and several firearms offenses after being shot and wounded by a Massachusetts State Police Trooper. At trial, the defendant's attorney argued that the defendant did not have a gun, fired at the defendant without justification, and then placed a gun where the defendant was laying.
In closing arguments, the prosecutor made statements to the jury that the defendant was a "street thug" and even went so far as calling the defendant's attorney a liar; and the defendant's theory of defense a "sham".
An except of the prosecutor's closing went like this:
"The entire defense in this case, I'd suggest to you, is a sham."
"[what about] the wad of money in his front pocket. Of course, all us unemployed people have a big wad of money in our pocket. Where's my money? They are street thugs who are out, and that they're going to do with those guns, luckily, we didn't get a chance to find out."
"It's the arrogance of street thugs that gets you in this case."
"As you look over all of this evidence...[it] will be obvious to you that the lies came from [the defense] table. And I'm not leaving out the attorney either..."
The defendant's convictions for the crimes of assault with intent to murder and firearms offenses were reversed by the Massachusetts SJC as a result of these improper comments made by the prosecutor to the jury.
In Massachusetts criminal trials, prosecutor are permitted to argue forcefully for a conviction based on the evidence, but it is improper to refer to: the defendant's election to not testify; misstate evidence or refer to facts not in evidence; interject personal belief in the defendant's guilt; play on racial, ethnic, or religious prejudice; play on the jury's sympathy or emotions; or comment on the consequences of a verdict.
Aside from being completely unprofessional, a prosecutor may also certainly not personally disparage the attorney or infer that the attorney is somehow misleading the jury.
In the same way, it is improper for prosecutors to repeatedly refer to the defendant as a "street thug", as this characterization encourages the jury to find the defendant guilty by virtue of his purported association with known criminals. Particularly in cases where there was no evidence of any such illicit associations, these arguments are extremely prejudicial to the defendant and may rise, as in this case, to the level of requiring reversal of the convictions.