WASHINGTON -- In the movie "In the Line of Fire," a deranged killer smuggles a homemade gun past the Secret Service and tries, unsuccessfully, to shoot the president.
That situation might have seemed far-fetched when the film, which starred Clint Eastwood as the agent who dives in front of the assassin's bullet, came out in 1993. But today, police officials and members of Congress fear that if a law known as the Undetectable Firearms Act is not renewed and updated when it expires Dec. 9, firearms that can slip past metal detectors and X-ray machines will become a law enforcement problem across the country.
It is not an idle concern: Homemade plastic guns are a reality, made possible by the proliferation of 3-D printing technology that was only getting started when the law was first passed by Congress and signed by President Ronald Reagan in 1988.
"They are so frightening because they render most standard detection useless," said Tim Murphy, a former deputy director of the FBI.
The expiring law bans guns that can pass unnoticed through a metal detector, and has been renewed twice in the 25 years since it was first enacted. But with the expiration date a little more than a week away, reauthorizing it has been caught up in a political standoff that has thwarted other recent attempts to enact gun safety legislation.
"We're on the clock, and as we know, this Congress doesn't deal well with deadlines," said Dan Gross, president of the Brady Campaign to Prevent Gun Violence.
For now, the extension is delayed as lawmakers fight over whether to simply extend the law or amend it to include new provisions aimed specifically at 3-D printed weapons.
Shortly before the Senate broke for its Thanksgiving recess, it set aside a measure to extend the law for a year because of objections by Republicans. Sen. Jeff Sessions, R-Ala., said he was concerned not about extending the law as written but about senators who support gun safety measures using the law as a backdoor way of attaching more provisions when it expires again.
"They're considering altering it, putting more language in it," Mr. Sessions said. "There's concern that it may be altered in a way that would be problematic."
The House is expected to approve a 10-year extension of the law when it returns next week. Democrats, led by Rep. Steve Israel of New York, want to include the 3-D weapons provision, but Republicans have agreed only to consider renewing the law as it is now.
"It's hard to believe that anyone would oppose a piece of legislation like this, so tied into, so connected with our safety," said Sen. Charles E. Schumer, D-N.Y., one of the senators leading the effort to extend the law. "In a world of terrorism, to say that we would make legal guns that can pass through metal detectors so people can slip them through airports, stadiums, schools?"
The National Rifle Association, whose position will carry significant weight with many in Congress, has not publicly signaled where it stands. A spokesman for the NRA did not respond to several requests for comment.