LAS VEGAS — The 2020 Democratic presidential candidates on Wednesday expressed their collective support for sweeping new gun control measures, appearing at a forum in a city still grieving after a gunman massacred 58 concertgoers two years and one day ago.

The forum, sponsored by two gun control organizations and MSNBC, was intended as a platform for a policy discussion — but it quickly became an outlet for rising anxiety among Democrats who fear the country has grown numb to mass shootings, like the ones that took place during a single August weekend in El Paso and in Dayton, Ohio.

“It’s not an intellectual issue,” said Senator Cory Booker of New Jersey, speaking to an audience that included the families of some shooting victims. “This is living with a sense of urgency. We cannot wait. This is no time for an impotence of empathy. We cannot wait until this hell is upon your community for you to be activated in this fight.”

For six hours, the event was punctuated by emotional and sometimes graphic descriptions of the horror that fell upon Las Vegas on Oct. 1, 2017.