Even after the Newtown, Conn., massacre, Republican opponents of reasonable firearms restrictions in Congress blocked proposals for strengthened background checks and other steps to make future gun tragedies less likely. Now the question is whether anti-gun-control extremists will allow a federal ban on the manufacture, sale, import or possession of guns that are undetectable by metal detectors and X-ray machines to expire on Dec. 9.

When Congress first approved the Undetectable Firearms Act in 1988, and renewed it in 1998 and 2003, the possibility of undetectable plastic guns being taken onto planes and into government buildings where guns are prohibited was largely theoretical. Today, 3-D printing technology has reached a point where it is possible to cheaply create fully functional plastic handguns capable of firing multiple shots. A blueprint for creating such a gun, known as the Liberator, was downloaded more than 100,000 times when it was posted on the website of a group called Defense Distributed earlier this year.

Last week, the Bureau of Alcohol, Tobacco, Firearms and Explosives tested a version of the Liberator produced by the agency and found its firepower to be sufficient to “reach vital organs and perforate the skull.” The weapon’s design calls for a small amount of metal to be included, which makes it legal under current law. But the metal part is tiny and can be easily removed. Agency officials are concerned about the spread of undetectable guns as 3-D printers become more widely available.

Senator Charles Schumer, a New York Democrat, is working with two Democratic colleagues, Bill Nelson of Florida and Patrick Leahy of Vermont, the chairman of the Judiciary Committee, to pass an updated renewal measure that responds to law enforcement concerns. Representative Steve Israel, Democrat of New York, is pressing a similar bill in the House. The ban on undetectable guns was first signed into law by President Ronald Reagan and previous renewals have received bipartisan support. This time should be no different.