“America is absolutely awash with easily obtainable firearms,” one spokesman for Al Qaeda said in a 2011 recruitment video. “So what are you waiting for?”

Few places on earth make it easier than the United States for a terrorist to buy assault weapons to mow down scores of people in a matter of minutes. The horrific massacre in Orlando last weekend is only the latest example. And all this is made vastly easier by a gun lobby that has blocked sensible safety measures at every turn, and by members of Congress who seem to pledge greater allegiance to the firearms industry than to their own constituencies. There is a word for their role in this form of terrorism: complicity.

On Wednesday, Senate Democrats began a filibuster to force a vote on gun-control legislation. If Congress is serious about the threat of terrorists using guns, there are several steps it can take right away.

First, support reasonable efforts to close the so-called terror gap, which would make it harder for suspected terrorists to get their hands on a gun. In December, Congress considered legislation by Senator Dianne Feinstein, a Democrat, and Representative Peter King, a Republican, that would have given the F.B.I. the ability to prevent gun sales to people it had reason to believe might be connected to terrorism. The bill was based on a Bush administration proposal, and versions of it have been pushed for years, but Republicans on Capitol Hill, beholden to the National Rifle Association and other gun-rights organizations, voted it down.