WASHINGTON — Gun control legislation the Senate debates next month will include an expansion of federal background checks for firearms buyers, Majority Leader Harry Reid said Thursday in a victory for advocates of gun restrictions.

The announcement underscores that Democrats intend to take an aggressive approach in the effort to broaden the checks, currently required for transactions involving federally licensed firearms dealers but not private sales at gun shows or online.

President Barack Obama and many supporters of curbing guns consider an expansion of the system to private gun sales to be the most effective response lawmakers could take in the wake of December's elementary school massacre in Newtown, Conn. The system is designed to keep guns from criminals, people with serious mental problems and others considered potentially dangerous.

The overall gun measure will also include legislation boosting penalties for illegal gun trafficking and modestly expanding a grant program for school security, said Reid, D-Nev. Its fate remains uncertain, and it will all but certainly need Republican support to survive.

Reid said that during Congress' upcoming two-week break, he hopes senators will strike a bipartisan compromise on broadening background checks. Without a deal, he indicated the gun bill would include a stricter version approved this month by the Senate Judiciary Committee and authored by Sen. Chuck Schumer, D-N.Y., expanding the system to virtually all private gun transactions with few exceptions.

"I want to be clear: In order to be effective, any bill that passes the Senate must include background checks," Reid said in a written statement.

Opponents including the National Rifle Association say background checks are easily sidestepped by criminals and threaten creation of a government file on gun owners — which is illegal under federal law.

"We remain as committed as we have been to opposing gun bans. History shows us that gun bans don't work to reduce crimes," said Andrew Arulanandam, an NRA spokesman. He declined to comment on a potential compromise but said if the Senate considers Schumer's version of background checks, "We will do whatever we can to defeat it."

The NRA wants Congress to fund more armed guards at schools, step up prosecutions of people who file false gun applications and increase the background check system's access to state records of people with serious mental illness and other problems.

Sen. Charles Grassley of Iowa, top Republican on the Senate Judiciary Committee, said of Reid's announcement, "I don't know how the leader expects members to vote on an ever-changing piece of legislation that has yet to gain bipartisan support."

If not included in the overall gun bill, an expansion of background checks could have been offered as an amendment. But that would have likely meant it would have needed support from 60 of the 100 senators to prevail — a difficult hurdle for Democrats.

Including expanded checks in the gun legislation signals either of two courses by Democrats: A feeling that they can win bipartisan support for the measure, or a willingness to essentially challenge Republicans to reject the entire gun-control package and face the political consequences in next year's elections.

It also pleases gun control backers who have said that in response to the Newtown killings, they expect Congress to do more than toughen gun trafficking penalties and boost school safety aid.

"Senator Reid's announcement is a tremendous step and we recognize there is still a tough road ahead," said Dan Gross, president of the Brady Campaign to Prevent Gun Violence, adding that his group would activate supporters to contact lawmakers.

"The majority leader's been a pretty steady guide throughout, and this a good example," said Mark Glaze, director of Mayors Against Illegal Guns, a group New York City Mayor Michael Bloomberg helps lead.

Reid said during next months' debate, he will allow votes on amendments including an assault weapons ban, curbs on high-capacity ammunition magazines and mental health. There is wide-ranging agreement that many states poorly report mental health records to the federal background check system.

Days ago, supporters of gun restrictions suffered a blow when Reid decided to exclude a proposed assault weapons ban from the gun bill the Senate will debate.

Reid said the ban lacked the 60 votes it would need and including it would risk defeat of the entire package. The ban's sponsor, Sen. Dianne Feinstein, D-Calif., plans to offer the provision as an amendment that seems certain to lose.

In a hint of possible movement, Schumer and two other senators who have spent weeks searching for a bipartisan deal are considering several options, including requiring background checks and record keeping for private sales at gun shows and commercial sales online. It would exclude in-person, non-commercial transactions between people who know each other. The idea was described by a lobbyist and Senate aide who spoke on condition of anonymity to discuss private talks.

Other exclusions could include gun transactions between relatives and acquisitions by people with state-issued concealed carry permits, and there would be an online background check system for people in remote areas. Veterans officially determined to have some psychological problems would be given a way to appeal that decision, which would otherwise bar them from getting firearms.

Besides Schumer, the Senate's No. 2 Democratic leader, other senators involved are moderate Democrat Joe Manchin of West Virginia, who has an NRA A-rating for his votes, and moderate Republican Mark Kirk of Illinois.

Schumer has been insisting on record keeping for all private gun sales, saying the files are needed to keep the system effective. That led to stalemated talks with conservative leader Sen. Tom Coburn, R-Okla., who says the data would lead to federal records on gun owners.

On Thursday, Bloomberg stepped up pressure on Congress to expand background checks, saying it would save lives and win broad public support.

"The only question is whether Congress will have the courage to do the right thing, or whether they will allow more innocent people, including innocent children, to be gunned down," he said at a New York news conference.

"It's time for the political establishment to show the courage your daughter showed," said Vice President Joe Biden, standing beside Bloomberg and motioning to the nearby family of a substitute teacher among 26 first-graders and educators killed at Newtown.

Biden later sent a Tweet thanking Reid for his decision.

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