Markey, Blumenthal and other Democrats filed legislation that would prohibit the publication of a digital file online that allows a 3D printer to manufacture a firearm. Democrats have filed a separate bill to require that all guns have at least one significant component made of metal.
The measure is intended to ensure that even guns primarily made of plastic can be discovered by metal detectors.
In a suit filed Monday in Seattle, eight Democratic attorneys general asked a judge to block the federal government's settlement with Defense Distributed that allowed the company to make the plans available online.
People can use the blueprints to manufacture plastic guns using a 3D printer. But industry experts have expressed doubts that criminals would go to the trouble, since the printers needed to make the guns are very expensive, the guns themselves tend to disintegrate quickly and traditional firearms are easy to come by.
Unlike traditional firearms that can fire thousands of rounds in a lifetime, the 3D-printed guns normally last only a few rounds before they fall apart, experts say. They don't have magazines that allow the usual nine or 15 rounds to be carried; instead, they usually hold a bullet or two and then must be manually loaded afterward. And they're not usually very accurate.
Cody Wilson, the founder of Defense Distributed, first published downloadable designs for a 3D-printed firearm in 2013. The plans were downloaded about 100,000 times until the State Department ordered him to cease, contending the effort violated federal export laws since some of the blueprints were downloaded by people outside the United States.
The State Department reversed course in late June, agreeing to allow Wilson to resume posting the blueprints.
The company filed its own suit in Texas on Sunday, asserting that it's the victim of an "ideologically fueled program of intimidation and harassment" that violates the company's First Amendment rights.
Meanwhile, Defense Distributed agreed to block temporarily Pennsylvania residents from downloading the plans after state officials went to federal court in Philadelphia on Sunday seeking an emergency order. The company said it has also blocked access to users in New Jersey and Los Angeles.
Associated Press writer Lisa Marie Pane in Boise, Idaho contributed to this story.