WASHINGTON (AP) — 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.

President Barack Obama signed the law before midnight, using an auto pen as he traveled to Africa for ceremonies honoring the late South African President Nelson Mandela. The device Obama used to sign the bill has been used for the signatures of traveling presidents since the administration of President George W. Bush.

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.

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 at schools, airports and elsewhere.

Sen. Charles Grassley of Iowa, top Republican on the Senate Judiciary Committee, said Congress should extend the ban for a decade and study Schumer's more restrictive plan to make sure it doesn't interfere with technologies used by legitimate gun manufacturers.

He said the bill's recent introduction suggests that "the real objectives were things other than just getting an extension."

At a news conference later, Schumer said he had "no ulterior motive" in proposing to strengthen the ban and said he hoped to find compromise with Grassley in coming weeks.

"The bottom line is technology advances and it does good things and it does bad things," he said.

Underscoring the issue's political sensitivity, both of Monday's votes were on unanimous consent requests. That meant any single senator could scuttle the proposals by objecting.

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It also meant the votes were by voice and that 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 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.

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 — as well as the NRA — 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.

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.

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.