Unburdened by re-election worries and empowered by law to act without Congress, U.S. President Barack Obama could take action to improve background checks on gun buyers, ban certain gun imports and bolster oversight of dealers.

Prospects for gun control legislation intensified in the wake of the school massacre in Newtown, Conn., as more pro-gun rights lawmakers said on Monday they were open to the possibility while Obama and three cabinet members met at the White House to discuss the subject.

Having just won a second four-year term, Obama does not need to fear alienating voters who favor gun rights and he could press ahead without lawmakers on fronts where federal law enables executive action.

Speaking in Newtown, where a gunman on Friday killed 20 children and six adults in an elementary school, Obama vowed late on Sunday to "use whatever power this office holds" to try to prevent such massacres.

"Because what choice do we have? We can't accept events like this as routine," Obama said at Newtown High School.

His administration has the power to issue executive orders or new rules, options that Obama is likely to consider in combination with possible new laws.

The National Rifle Association, the largest U.S. gun rights group with 4 million supporters, relies largely on its ability to influence lawmakers in order to block legislation.

Obama's appointees at the U.S. Justice Department have been studying ideas since the Jan. 8, 2011, shooting of U.S. Representative Gabrielle Giffords of Arizona and 18 others at a public meeting. Giffords survived but six people died.

Christopher Schroeder, who ran the Justice Department's review, said it looked at possible legislation to send to Congress as well as action the administration could take itself.

"You always look at both, because if you can do it administratively it's certainly a less involved process," said Schroeder, who has since returned to a professorship at Duke Law School.

Many of the ideas have to do with the background checks that licensed gun dealers run on potential buyers.

Critics Cite Holes

Critics say the system has holes because it does not include all the data it should on those ineligible to buy guns. The FBI, which runs the system, could incorporate more data from within the federal government - using evidence of mental incompetence, for example.

(Read More: Why Gun Sales Often Rise After Mass Shootings)

There are privacy concerns, however, and the Justice Department is still studying which types of data it can legally use, Schroeder said.

"That kind of system works effectively only if all of the potentially disqualifying information that has been gathered by any federal, state or local authority is accessible to the database, and that's not the case today," he said.

It is not clear what changes to the background checks would have prevented the mass shooting in Newtown, because the killer appeared to have used weapons his mother bought legally.

Other proposals for executive action by Obama include sharing information with state and local law enforcement about possibly illegal purchases; maintaining data on gun sales for longer periods to help with investigations; and restricting the importation of certain military-style weapons, as President George H.W. Bush did in 1989.

A pro-gun control mayors' group co-chaired by New York City Mayor Michael Bloomberg has pushed the Obama administration since 2009 to adopt 40 recommendations it said were allowed under existing law.

One of the 40 has been put into effect, said Mark Glaze, director of Mayors Against Illegal Guns, and even that recommendation — requiring gun dealers to report sales of multiple semiautomatic weapons — drew heated resistance.

In 2011, when the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) adopted a version of the recommendation aimed at dealers in states near the U.S.-Mexico border, gun makers sued and congressional Republicans tried to eliminate funding for the rule.

A judge upheld it, allowing it to go into effect. The case is now on appeal.

Lobbying Blitz

Bloomberg's group is still pushing the other recommendations as it makes plans for a lobbying blitz over new laws, such as a ban on high-capacity magazines. "While they are important, they're not the big-ticket items. And we're in a big-ticket world," Glaze said.

The administration also has leeway to act in how it defines certain categories of people prohibited from buying a gun.

Federal law bars anyone "who has been adjudicated as a mental defective," but it does not specify whether that means only a court can disqualify someone, said Michael Volkov, a former Republican Justice Department official now at the law firm LeClairRyan.

Another option could be changing how long a firearms dealer must keep records of a sale - a period that is now three days but could be extended, Volkov said.

Since the Justice Department began reviewing ideas to prevent mass shootings in early 2011, it has implemented a handful of changes.

In May, the department unveiled an automated system to feed records of federal indictments into the background checks database, replacing a system in which prosecutors uploaded information manually.

Schroeder said the department's review of firearms-related ideas is ongoing. He described the process as informal, and not one that has produced a formal report.