In a court filing late Friday, Tsarnaev's lawyers say some charges are repetitive and that the number could sway jurors weighing whether to find him guilty and, if they do, whether to sentence him to death.
Three people were killed and more than 260 wounded in the April bombing. Tsarnaev is also charged in the killing of a campus police officer during a shootout days later.
More than half of the 30 federal charges carry a possible death sentence. Experts have said earlier filings suggest the defense may try to save Tsarnaev's life by arguing he fell under the influence of his brother. Friday's motion was submitted by his lawyer Judy Clarke, one of the nation's foremost death penalty specialists.