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 grade school massacre in Newtown, Conn.By voice vote, the Senate gave final congressional approval to a 10-year extension of the prohibition against 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, and the measure now goes to President Barack Obama for his signature.Obama, traveling to Africa for ceremonies honoring the late South African president Nelson Mandela, was expected to sign the bill before midnight using an auto pen, a White House official said. GOP senators rejected an effort by Sen. Chuck Schumer, D-N.Y., 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.“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 “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 re-election next year in GOP-leaning states, the day’s votes could have been difficult.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 first-graders 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 just 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 Sen. Chris Murphy, D-Conn. “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 have not opposed renewing the current prohibition. The House approved a 10-year extension last week.Plastic guns were in their infancy when President Ronald 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.Sen. Charles Grassley of Iowa, top Republican on the Senate Judiciary Committee, says that with the law’s expiration at hand, Congress should extend it for a decade and study Schumer’s more restrictive plan later.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 prevent criminals and mentally ill people from getting 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, Rep. Tim Murphy, R-Pa., a psychologist, plans to announce legislation Thursday aimed at boosting federal mental health programs, including treatment, research and training for workers who respond to emergencies.The lack of movement in the Democratic-led Senate has left gun-control groups divided about their 2014 goals.Some are willing to set aside, for now, the push for expanded background checks and settle for more modest changes. These could include strengthening mental health programs and having states provide more records to the federal background check system.Backing this approach are some Newtown families and the group Americans for Responsible Solutions, formed by former Rep. Gabrielle Giffords, D-Ariz., seriously wounded by a shooter, and her husband, former astronaut Mark Kelly.“Anything we can do, anything Congress can do to potentially save a life, one life, they should do,” said Pia Carusone, executive director of Giffords’ group.Others want to continue raising pressure on lawmakers to back strong background check requirements, and they oppose aiming for less.These groups include Mayors Against Illegal Guns, led by outgoing New York City Mayor Michael Bloomberg, an organization that has been spending money against gun-rights congressional candidates and lawmakers. These groups are concerned that Republicans would use votes for weaker efforts to cast themselves as having championed major steps against guns.“Our interest in giving (New Hampshire GOP Sen.) Kelly Ayotte a vote on a mental health bill, which would be a good bill but do virtually nothing to solve the gun violence problem in this country, approaches zero,” said Mark Glaze, the mayors group executive director.Ayotte voted against expanded background checks in April and has been criticized by gun-control groups ever since.