After Newtown, there was tremendous confidence that commonsense gun control legislation would prevail. And for a while everything seemed to be on track. Two pro-gun senators, Republican Patrick J. Toomey of Pennsylvania and Democrat Joe Manchin of West Virginia, had teamed up to write a bill, and they had reached a compromise that would have expanded background checks to cover more private firearms sales. Endorsed by the White House and pushed by the Democratic leadership, this modest package died on the Senate floor last Tuesday, succumbing (along with a revived assault weapons ban) to a Republican-led filibuster that attracted crucial Democratic support. In a week already filled with sorrow, Tuesday was a day of shame.

There is plenty of blame to go around and, as we write post-mortems, President Obama and Senate Majority Leader Harry Reid must share in responsibility for defeat. The president made compelling arguments and intense emotional appeals for firearms-safety legislation, and his erudition won over the American people—90 percent of whom supported the proposal. He slipped up at the inside game, though, and having neglected to twist arms was left to wring hands and wipe tears. Reid equivocated on the assault weapons ban and passed up a recent opportunity for real filibuster reform, something that may have predestined the bill for legislative limbo.

That said, most criticism must be directed at the 45 senators who voted against cloture, especially the four Democratic defectors—Mark Pryor of Arkansas, Heidi Heitkamp of North Dakota, Max S. Baucus of Montana, and Mark P. Begich of Alaska—who cravenly decided their political capital was more important than human lives. Contrary to appearance, it was not their seats that were at risk. Most of their constituents agree with expanded background checks and the closure of the gun-show loophole, making their vote against cloture more infuriating.

By their cowardice, these Capitol Hill minions of the gun lobby have done nothing to prevent the next Jared Lee Loughner, James Holmes, or Adam Lanza to play out his homicidal passions in the nation’s stores, movie theaters, or classrooms. We thought Congress would have followed the example of legislators in states like Connecticut, who responded to the Newtown tragedy with appropriate reform. We did not expect so many to fall behind the National Rifle Association, a group that personifies the outsize power that special interest groups wield over American lawmaking. We do not understand how they could look into the faces of gun violence victims and their family members and vote the way they did. They will have to answer for their actions. We await the day with dread.

CORRECTION: April 23, 2013

An earlier version of this editorial stated that Senator Mark R. Warner voted against cloture last week. In fact, Senator Warner voted yes on the measure. The fourth Democratic senator to vote no was Mark Pryor of Arkansas.