Talks are intensifying in the Senate on compromise legislation to stop gun sales to people on some terrorism watch lists.
It comes as Democrats shut down the House of Representatives with a sit in at the front of the chamber on Wednesday (local time) to protest their chamber's lack of action.
Several dozen US House Democrats pushing for action on gun control protested on the floor of the House, chanting "no bill, no break!" and demanding that the chamber put off an upcoming recess until legislation is debated.
The protest was the latest move by Democrats to persuade the Republican majority in Congress to take up gun control in response to last week's mass shooting in Orlando, Florida, the deadliest in modern US history.
Assistant Democratic Leader James Clyburn, likened the push for gun control to the civil rights movement of the 1960s when sit-ins and other civil disobedience prodded Washington to act on new protections for African-Americans.
Democratic presidential candidate Hillary Clinton tweeted her support for the sit-in, saying, "This is what real leadership looks like."
Democrats in the Senate last week took control of that chamber for nearly 15 straight hours as they called for gun control legislation.
On Monday (local time), the Senate failed to advance four gun measures, including one that would have prohibited gun sales to people on a broad range of government watch lists.
Senators from both parties now are pushing for a compromise.
The powerful National Rifle Association (NRA) said it opposes the compromise bill and called it unconstitutional.
House Republicans have declined to advance gun control legislation and House Speaker Paul Ryan is "waiting to see what the Senate does" before discussing the topic.
The Orlando gunman, Omar Mateen, pledged allegiance to Islamic State during the June 12 rampage in which he killed 49 people and wounded 53 with an assault rifle and pistol at a gay nightclub before being fatally shot by police.