U.S. District Judge George A. O'Toole Jr. said it is inappropriate to make distinctions between those who are naturalized as U.S. citizens and those born in the country. Prosecutors described Tsarnaev's alleged violation of the oath he took when he became a citizen as betraying the country.
"It's unduly prejudicial, and I will strike," O'Toole said during a status conference.
Tsarnaev was a 19-year-old student at the University of Massachusetts Dartmouth when he and his older brother, Tamerlan, allegedly set off two bombs near the finish line of the Boston Marathon on April 15, 2013, killing three people and wounding many more. Tamerlan was killed in a confrontation with police a few days later -- a few hours after he and Dzhokhar allegedly killed a Massachusetts Institute of Technology police officer, Sean Collier.
The brothers were born in the Soviet Union. Tamerlan was a legal U.S. resident, while Dzhokhar became a citizen in 2012.
O'Toole said prosecutors have plenty of grounds for the death penalty that are "not obnoxious." He said using the oath as grounds fails that test because those who are natural-born citizens do not have to take one.