Another attempt at gun control fell short in the U.S. Congress on Thursday despite outrage at the Orlando massacre, as senators failed to back a proposed ban on firearms sales to people being monitored for links to terrorism.

It was the latest setback for proponents of gun restrictions who have been thwarted for years on Capitol Hill by gun rights defenders and the National Rifle Association.

A few hours after Democratic lawmakers ended a rowdy sit-in protest in the House of Representatives over guns, Senate Republican leaders closed a protracted debate over gun control, until at least after the July 4 holiday.

They argued that Sen. Susan Collins, a Republican from Maine, could not garner enough support for a proposal to forbid gun sales to anyone on the U.S. government's "No Fly List" for terrorism suspects or the "Selectee List" of people who receive extra security screening at airports.

“Eventually this problem will get addressed again one of two ways: We find a breakthrough, which I will seek, or there will be another terrorist attack which will bring us right back to this issue. I hope we can do it without another terrorist attack,” said Senator Lindsey Graham, a Republican who supported Collins.

The many efforts at gun control have failed despite anger at mass shootings like the killings at an elementary school in Newtown, Connecticut, in 2012 and in San Bernardino, California, last year.

After a gunman pledging allegiance to the Islamic State group fatally shot 49 people at a gay nightclub in Orlando earlier this month, some senators had seen resistance to gun restrictions softening because the issue had partly become one of national security.

But Collins' measure received only 52 votes in the 100-seat Senate in a procedural vote, and Republican leaders said she could not win the 60 votes needed for full approval. Four other gun control measures failed earlier this week.

"We have grieved for too many children and wept for too many families after shootings. Chicago. Tucson. Newtown. Charleston. Orlando. #Enough," first lady Michelle Obama wrote on Twitter on Thursday.