IOWA CITY, Iowa — The 2012 massacre at Sandy Hook Elementary School, in which a mentally troubled young man killed 26 children and teachers, served as a rallying cry for gun-control advocates across the nation.

But in the three years since, many states have moved in the opposite direction, embracing the National Rifle Association's axiom that more "good guys with guns" are needed to deter mass shootings.

In Kansas, gun owners can now carry concealed weapons without obtaining a license. In Texas, those with permits will soon be able to carry openly in holsters and bring concealed weapons into some college classrooms. And in Arkansas, gun enthusiasts may be able to carry weapons into polling places next year when they vote for president.

Dozens of new state laws have made it easier to obtain guns and carry them in more public places and made it harder for local governments to enact restrictions, according to a review of state legislation by The Associated Press. The number of guns manufactured and sold and the number of permits to carry concealed weapons have also increased, data show.

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The trend has been discouraging to some gun-control advocates, even as other states have adopted stricter background checks. Other gun-control supporters say their movement is emboldened by the recent rise of Everytown for Gun Safety, a well-funded group backed by former New York City Mayor Michael Bloomberg that is becoming influential in some state capitols.

The debate over gun rights moved to states after Congress rejected a bill in 2013 that would have expanded background checks to all gun sales, including those at gun shows and over the Internet. The arguments are expected to intensify next year as legislatures convene in the wake of the mass shooting of county government employees in San Bernardino, California, which is being investigated as an act of terrorism.

Recent mass shootings at a Planned Parenthood clinic in Colorado, a community college in Oregon and a church in South Carolina have also reignited passions on both sides.

"Most of our churches are just wide open," said Mississippi Republican Rep. Andy Gipson, who plans to file a bill next year allowing congregations to designate people who could carry guns.

The pro-gun legislation reflects a growing public sentiment that "gun-free zones are magnets for bad guys," said David Kopel, a gun policy expert at the Independence Institute, a libertarian think tank in Colorado. He said that concept was not popular after the 1999 shooting at Columbine High School in Colorado, but the frequency of mass shootings since then has made the idea of having a trained, law-abiding gun owner present more appealing.

"We've gone from, 'You can't even say that out loud' to it being an evenly divided issue, with the pro-gun side having an advantage on that," he said. "I would expect that we will see continued movement on that in the coming year."

Even before the Dec. 2 shooting at the office holiday party in San Bernardino, gun purchases and permit applications were on the rise.

On the day after Thanksgiving this year, U.S. gun sales approached a single-day record. More than 185,000 federal background checks were initiated, the most in the 17-year history of the program, according to FBI data.

"Everybody is swamped," said Mike Conway, a salesman at Bullseye Sport in Riverside, California, near San Bernadino, which has run out of most guns. "A lot of first-time buyers. A lot of people that realize that they have to be responsible for their own safety."

From 2007 to 2014, the number of concealed-carry handgun permits in states nearly tripled, from 4.7 million to 12.8 million, according to a recent report by the Crime Prevention Research Center, a group whose research is often cited by gun-rights supporters. Meanwhile, several states have passed laws shielding the identities of permit holders to protect privacy and prevent potential harassment.

Instead of limiting access to firearms after Sandy Hook, states such as Indiana and Mississippi passed laws to beef up the presence of police officers in schools. Kansas adopted a law allowing people to carry concealed weapons in many public buildings. Georgia and Arkansas, among others, allowed concealed weapons in bars and some churches. Tennessee made clear that permit holders can carry concealed weapons in vehicles and parks.

Several states also passed reciprocity agreements recognizing gun permits approved by other states, reduced permitting fees and loosened requirements. Wisconsin, for instance, eliminated a 48-hour waiting period to buy handguns.

And then there are new laws designed to thwart gun-control measures. States have prohibited authorities from seizing guns during emergencies, moved to ban the use of taxpayer funding for government gun buyback programs and banned the destruction of firearms seized by law enforcement. Some Republican-controlled states have pre-empted local governments' ability to pass stricter firearms laws by declaring that it's a matter for the state.

Everytown President John Feinblatt said many of the measures that expanded gun rights were passed when the NRA faced little opposition in statehouses, but that is starting to change. He said his group succeeded this year in opposing bills in several states that would have allowed concealed weapons on college campuses and permitted people to carry without obtaining permits.

Since Sandy Hook, six states have expanded background checks, and two more such measures are expected to be on statewide ballots next year in Nevada and Maine, Feinblatt said. His group, he added, isn't concerned with how many guns exist, but wants rules in place to make sure they aren't sold or transferred to criminals and the mentally ill.

"If more responsible gun owners want more guns and they are doing it the right way, that's not going to affect public safety," he said.

Eric Fleegler, a doctor at Boston Children's Hospital who has studied state gun laws, said he worries that the expansion of gun rights could cause more fights to escalate into deadly confrontations, more people to commit suicide and more kids to die from gun accidents.

"In a country with 330 million people and 310 million guns," he said, "the suggestion that the problem is we don't have enough guns available just doesn't seem to hold much weight."