Democratic presidential hopeful Elizabeth Warren is calling on Walmart to stop selling guns.

The Massachusetts senator is joining a push by gun control activists following two mass shootings that killed 31 last week. One of those shootings took place at a Walmart in El Paso, Texas, and 22 people were killed.

Warren says on Twitter that Walmart (WMT) "is one of the largest gun retailers in the world" and has a responsibility to prioritize public safety.

Walmart last year announced it would cease selling guns and ammunition to anyone under age 21. It also removed assault-style rifles from its website. A Walmart spokesman says "we are going through a thorough review of our policies" following the shooting.

Warren is campaigning in Iowa this weekend, along with other White House candidates.