WASHINGTON (AP) — A Senate vote to renew an expiring ban on plastic firearms capable of evading metal detectors and X-ray machines is shaping up as a bittersweet moment for gun control supporters, days before the anniversary of the deadly mass shooting at Sandy Hook Elementary School in Connecticut.

Monday's vote to extend the prohibition on plastic guns for another decade responds to a growing threat from steadily improving 3-D printers that can produce such weapons. But gun control advocates seem sure to lose an effort to impose additional, tougher restrictions on plastic firearms — a harsh reminder of their failure to enact any new federal gun curbs in the year since 20 first-graders and six educators were murdered in Newtown, Conn.

The slayings last Dec. 14 prompted the newly re-elected President Barack Obama to push gun control to the top of his domestic agenda. But Congress approved nothing, and gun control advocates face the same uphill struggle in 2014, complicated by internal divisions over what their next step should be.

"The gun lobby still has enormous power in Washington — more, frankly, than I thought they still had," said Sen. Chris Murphy, D-Conn., who represented Newtown last year while in the House.

Illustrating the roadblocks that have thwarted gun control forces, an effort by Sen. Chuck Schumer, D-N.Y., to make plastic guns more detectable by requiring them to have a permanent metal part seems certain to fail Monday. His plan is opposed by Republicans and the National Rifle Association.

The Senate is then expected to easily approve a 10-year extension of the ban, which would otherwise expire Tuesday.

Schumer and other Democrats, as well as gun-control advocates and law enforcement officials, say there's a problem with current law on plastic guns: It lets gun makers meet its requirements by including a metal part that can be easily detached — thus letting the weapon evade screening devices.

In a statement last week, the NRA expressed no opposition to renewing the law. But the gun lobby said it would fight any expanded requirements, including Schumer's "or any other proposal that would infringe on our Second Amendment rights" to bear arms.

The prohibition was first enacted in 1988 under President Ronald Reagan and easily renewed twice. The House approved a 10-year extension of the ban last Tuesday.

Sen. Charles Grassley of Iowa, top Republican on the Senate Judiciary Committee, says that with the law's expiration at hand, Congress should quickly enact a long extension and study Schumer's plan later. Other Republicans agree.

Supporters of tightening the rules say the 10-year renewal plays into the NRA's hands because it reduces Democrats' ability to revisit the issue.

If, as expected, Democrats fail Monday to tighten the restrictions, it will be the latest in a series of setbacks this year.

Their biggest defeat came in April, when the Senate blocked an effort to expand required background checks for firearms buyers. The proposal was Obama's top gun-control priority following the elementary school killings.

Background checks, aimed at preventing criminals and the mentally ill from getting weapons, are currently required only for purchases from licensed gun dealers. The rejected bill, by Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., would have extended the requirement to all guns bought on the Internet and at gun shows.

Also rejected in April were proposals to ban assault weapons and large-capacity ammunition magazines, which have been used in mass shootings.

House Republican leaders never favored any of those proposals, and none came to a vote there.

But with Saturday's Newtown anniversary drawing attention to the issue, Rep. Tim Murphy, R-Pa., a psychologist, plans to announce legislation Thursday aimed at bolstering federal mental health programs, including treatment, research and training for workers who respond to emergencies.

In the Senate, Majority Leader Harry Reid, D-Nev., has gained none of the five new votes he would need to prevail on background checks.

Eager to avoid exposing potentially vulnerable Democrats seeking re-election next year to politically fraught votes, Reid has said he won't revisit the issue until he can win.

That has left gun control groups split over strategy.

Some want to pursue more modest improvements like strengthening mental health programs and broadening the scope of reports that states provide to the federal background check system.

Groups backing this approach include some Newtown families and Americans for Responsible Solutions, formed by former Rep. Gabrielle Giffords, D-Ariz., seriously wounded by a mass shooter, and her husband, former astronaut Mark Kelly.

"We see only good coming from passing mental health legislation around which there is broad bipartisan agreement," said a statement from Sandy Hook Promise, a group representing some Newtown families. "If we don't begin to bury at least some of our differences, we will continue to needlessly bury our children."

Others want to continue raising pressure on lawmakers to back strong background check requirements, and oppose settling for less.

These groups include Mayors Against Illegal Guns, led by outgoing New York City Mayor Michael Bloomberg, which has been spending money against gun-rights congressional candidates and lawmakers. They fear 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.

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  • Rep. Louie Gohmert (R-Texas)

    "I wish to God she had had an m-4 in her office, locked up so when she heard gunfire, she pulls it out ... and takes him out and takes his head off before he can kill those precious kids," Gohmert said of slain principal Dawn Hochsprung on <a href="http://www.huffingtonpost.com/2012/12/16/louie-gohmert-guns_n_2311379.html"><em>Fox News Sunday</em></a>. He argued that shooters often choose schools because they know people will be unarmed.

  • Virginia Gov. Bob McDonnell (R)

    "If people were armed, not just a police officer, but other school officials that were trained and chose to have a weapon, certainly there would be an opportunity to stop an individual trying to get into the school," he <a href="http://www.wtop.com/610/3162096/Gov-Is-it-time-to-arm-school-officials">told WTOP's "Ask the Governor" show</a> Tuesday, warning that Washington may respond to such a policy with a "knee-jerk reaction."

  • Tennessee Gov. Bill Haslam (R) & State Sen. Frank Niceley (R)

    Gov. Haslam says he will consider a Tennessee plan to secretly arm and train some teachers, <a href="http://tpmdc.talkingpointsmemo.com/2012/12/tennessee-armed-teachers.php">TPM reports</a>. The legislation will be introduced by State Sen. Frank Niceley (R) next month. "Say some madman comes in. The first person he would probably try to take out was the resource officer. But if he doesn’t know which teacher has training, then he wouldn’t know which one had [a gun]," Niceley told TPM. "These guys are obviously cowards anyway and if someone starts shooting back, they’re going to take cover, maybe go ahead and commit suicide like most of them have."

  • Oklahoma State Rep. Mark McCullough (R) & State Sen. Ralph Shortey (R)

    State Rep. Mark McCullough (R) <a href="http://www.tulsaworld.com/news/article.aspx?subjectid=336&articleid=20121217_336_0_OKLAHO168827">told the Tulsa World</a> he plans to file legislation that would bring guns into schools, calling their absence "irresponsible." “It is incredibly irresponsible to leave our schools undefended – to allow mad men to kill dozens of innocents when we have a very simple solution available to us to prevent it," he said. "I’ve been considering this proposal for a long time. In light of the savagery on display in Connecticut, I believe it’s an idea whose time has come." Sen. Ralph Shortey (R) told the Tulsa World that teachers should carry concealed weapons at school events. "Allowing teachers and administrators with concealed-carry permits the ability to have weapons at school events would provide both a measure of security for students and a deterrent against attackers," he said.

  • Florida State Rep. Dennis Baxley (R)

    Baxley, who once sponsored Florida's controversial Stand Your Ground law, <a href="http://politics.heraldtribune.com/2012/12/17/florida-legislator-allow-guns-in-schools/">told the Sarasota Herald-Tribune </a>that keeping guns out of schools makes them a target for attacks. “We need to be more realistic at looking at this policy," he said. "In our zealousness to protect people from harm we’ve created all these gun-free zones and what we’ve inadvertently done is we’ve made them a target. A helpless target is exactly what a deranged person is looking for where they cannot be stopped.”

  • Texas Gov. Rick Perry (R)

    At a Tea Party event Monday night, <a href="http://www.huffingtonpost.com/2012/12/18/rick-perry-guns-in-schools_n_2322185.html">Perry praised a Texas school system that allows some staff to carry concealed weapons to work</a> and encouraged local school districts to make their own policies.

  • Minnesota State Rep. Tony Cornish (R)

    Cornish <a href="http://www.kdlt.com/index.php?option=com_content&task=view&id=22736&Itemid=57">plans to introduce legislation that would allow teachers to arm themselves</a>, according to the AP.

  • Oregon State Rep. Dennis Richardson (R)

    In an email <a href="http://www.huffingtonpost.com/2012/12/17/oregon-state-rep-dennis-richardson-teacher-guns-stopped-connecticut-shooting_n_2317444.html?ir=Education">obtained by Gawker</a> and excerpted below, Richardson tells three superintendents that he could have saved lives had he been armed and in Sandy Hook on Friday: <blockquote>If I had been a teacher or the principal at the Sandy Hook Elementary School and if the school district did not preclude me from having access to a firearm, either by concealed carry or locked in my desk, most of the murdered children would still be alive, and the gunman would still be dead, and not by suicide. ... [O]ur children's safety depends on having a number of well-trained school employees on every campus who are prepared to defend our children and save their lives?</blockquote>

  • Former Education Secretary Bill Bennett

    "And I'm not so sure -- and I'm sure I'll get mail for this -- I'm not so sure I wouldn't want one person in a school armed, ready for this kind of thing," Bennett, who served as education secretary under Ronald Reagan, <a href="http://www.huffingtonpost.com/2012/12/16/bill-bennett-education-secretary-connecticut-shooting_n_2311774.html">told <em>Meet the Press</em> Sunday</a>. "The principal lunged at this guy. The school psychologist lunged at the guy. It has to be someone who's trained, responsible. But, my god, if you can prevent this kind of thing, I think you ought to."