WASHINGTON — Narrowly beating a midnight deadline, Congress voted Monday to renew an expiring ban on plastic firearms that can evade airport detection machines. But Republicans blocked an effort to toughen the restrictions — the latest defeat for gun-control forces in the year since the school massacre in Newtown, Conn.
By voice vote, the Senate gave final congressional approval to a 10-year extension of the prohibition against the lethal plastic guns that can slip past metal detectors and X-ray machines.
The House voted last week for an identical decade-long renewal of the ban.
But GOP senators rejected an effort by Senator Chuck Schumer, Democrat of New York, to strengthen the ban by requiring that such weapons contain undetachable metal parts.
Some plastic guns meet the letter of the current law with a metal piece that can be removed, making them a threat to be slipped past security screeners at schools, airports, and elsewhere.
‘‘Who in God’s name wants to let plastic guns pass through metal detectors at airports or stadiums?’’ Schumer said in an interview Monday.
The National Rifle Association, which has been instrumental in blocking gun restrictions, expressed no opposition to renewing the law. But the gun lobby said it would fight any expanded requirements, including Schumer’s, ‘‘that would infringe on our Second Amendment rights’’ to bear arms.
Underscoring the issue’s political sensitivity, both of Monday’s votes were by voice only, meaning no individual senators’ votes were recorded. For a handful of Democratic senators seeking reelection next year in GOP-leaning states, recorded votes could have been difficult.
President Obama, who is traveling to South Africa, is expected to sign the bill before midnight using an auto pen, the White House said.
The rejection of stricter curbs highlighted the repeated setbacks for gun-control advocates in Congress since last Dec. 14. On that day, a gunman fatally shot 20 pupils and six staff members at Sandy Hook Elementary School before killing himself.
Despite that — and other recent mass shootings, including at the Washington Navy Yard blocks from the Capitol — supporters of expanded gun control are nearing the end of a year in which they have been unable to push any new firearms restrictions through Congress.
‘‘We’re several decades behind the NRA,’’ said Senator Chris Murphy, Democrat of Connecticut. “This is a long game, and it’s going to take us some time to build up the resources necessary to compete.’’
Congressional Republicans have resisted tightening the restrictions against undetectable guns, but those lawmakers — as well as the NRA — have not opposed renewing the current prohibition.
Plastic guns that are not toys were uncommon when President Reagan and Congress first enacted the ban against undetectable firearms and when it was renewed in 1998 and 2003. But such weapons have become a growing threat and can now be produced by 3-D printers, which are becoming better and more affordable.
Senator Charles Grassley of Iowa, top Republican on the Senate Judiciary Committee, said Schumer’s more restrictive plan should be studied.
Supporters of tightening the rules say the 10-year renewal helps the gun lobby because it reduces Democrats’ ability to revisit the issue.
The Sandy Hook killings prompted Obama and Democrats to make gun control a top domestic priority this year — but to no avail in Congress.
Their most stinging loss occurred in April, when the Senate turned aside an effort to expand federal background checks for would-be gun purchasers, an effort to make it harder for criminals and mentally ill people to get weapons.
That measure would have required the checks for all sales at gun shows and online — expanding a system that is currently required only for sales through licensed firearms dealers.
Also defeated were proposed bans on assault weapons and large-capacity ammunition magazines.
In a measure of GOP opposition and NRA clout, those proposals never even came to votes in the Republican-majority House.
But with Saturday’s Newtown anniversary approaching, Representative Tim Murphy, Republican of Pennsylvania and a psychologist, plans to announce legislation Thursday aimed at boosting federal mental health programs, including training for workers who respond to emergencies.