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 transactions 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.

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.

On Thursday, New York City Mayor Michael Bloomberg, a leader of hundreds of mayors seeking stricter gun curbs, stepped up pressure on Congress to expand background checks, saying there 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.

Tougher penalties for illegal firearms trafficking and expanded school safety grants both received bipartisan support when they were approved this month by the Senate Judiciary Committee, and both are considered effective by gun curb advocates. But some supporters of firearms restrictions say Congress should do more.

“Inadequate for the moment,” said Mark Glaze, director of Mayors Against Illegal Guns, the group Bloomberg helps lead.