The announcement underscores that Democrats intend to take an aggressive approach in the effort to broaden the checks, currently required only for transactions involving federally licensed firearms dealers.

President Barack Obama and many supporters of curbing guns in and out of Congress 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 the Senate debates will also include legislation boosting penalties for illegal gun trafficking and modestly expand a grant program for school security, said Reid, D-Nev.

Reid said that during Congress' upcoming two-week break, he hopes senators will strike a bipartisan compromise on broadening background checks. But without a deal, he indicated the gun bill would include a stricter version approved this month by the Senate Judiciary Committee 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.

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.