Another bill that would have expanded background checks to gun show and online firearms sales to screen out convicted felons and the mentally ill also failed on Thursday. The four Republican senators running for president — Ted Cruz, Marco Rubio, Rand Paul and Lindsey Graham — all turned up to vote against these common-sense measures.

“If you need proof that Congress is a hostage to the gun lobby, look no further than today’s vote,” said Senator Dianne Feinstein, who sponsored the terror watchlist measure.

In the hours after the attack in San Bernardino on Wednesday, President Obama specifically mentioned that legislation as an important security measure. “Those same people who we don’t allow to fly can go into a store in the United States and buy a firearm, and there’s nothing that we can do to stop them. That’s a law that needs to be changed,” he said on CBS News. The George W. Bush administration backed the terrorist-list bill in 2007.

No matter. The House speaker, Paul Ryan, issued his party’s weak defense of arming potential terrorism suspects on Thursday morning: “I think it’s very important to remember people have due process rights in this country, and we can’t have some government official just arbitrarily put them on a list.” Mr. Ryan’s Senate colleagues demonstrated that they are more worried about the possibility that someone might be turned away from a gun shop than shielding the public against violent criminals.

At the Republican Jewish Coalition’s conference on Thursday, the Republican presidential candidates offered little but political attacks. Senator Cruz immediately blamed Mr. Obama: “Coming on the wake of the terror attack in Paris, this horrific murder underscores that we are at a time of war, whether or not the current administration realizes it or is willing to acknowledge it, our enemies are at war with us and I believe this nation needs a wartime president to defend it.”