WASHINGTON — After a gunman in Las Vegas slaughtered country-music fans with assault weapons that mimicked a machine gun, lawmakers from both parties said they would move quickly to ban so-called bump stocks.

After a mass shooter massacred churchgoers in Sutherland Springs, Tex., the No. 2 Republican in the Senate, John Cornyn of Texas, was stirred to action, pledging legislation to bolster the nation’s instant background check system for would-be gun buyers.

Congress has effectively done neither.

That inaction sank in on Thursday, as lawmakers confronted yet another mass shooting, this time with some of the survivors imploring Congress to finally take action. Republicans called for prayers, but argued that no single fix to the nation’s gun laws would deter a shooting like the one on Wednesday in Parkland, Fla., that left 17 people dead, many of them children.

“This is one of those moments where we just need to step back and count our blessings,” Speaker Paul D. Ryan of Wisconsin said. “We need to think less about taking sides and fighting each other politically, and just pulling together.”