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.

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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.