LAS VEGAS—Democratic presidential candidates and gun-control advocates are divided on whether the government should mandate buybacks of assault-style weapons, with some fearing it could jeopardize goals such as broadening background checks.

More than a dozen progressive activists who attended the Giffords/March for Our Lives gun safety forum, which attracted nine presidential candidates on Wednesday, said in interviews they favored immediate action in Washington on issues where there is clearer consensus.

“We’re all having the same conversation: to save lives…and have common-sense gun legislation,” said Stephanie Pizzoferrato, a teacher in Las Vegas and a member of Moms Demand Action. Ms. Pizzoferrato, whose daughter died from a stray bullet, said she didn’t want a debate over gun buybacks to “overshadow the policies we can see immediately.”

Some of the leading progressive gun-control advocacy groups, including Giffords—named for former Rep. Gabrielle Giffords (D., Ariz.), the survivor of a 2011 shooting—and Moms Demand Action, have stopped short of supporting mandatory buybacks. Democratic leaders in Congress aren’t there yet either. But Republicans—who say such a program would infringe on Second Amendment rights—have focused on recent comments by former Texas Rep. Beto O’Rourke, a low-polling presidential candidate who called for mandatory buybacks of assault-style weapons during a debate last month.

President Trump seized on those remarks. “Dummy Beto made it much harder to make a deal. Convinced many that Dems just want to take your guns away. Will continue forward!,” Mr. Trump tweeted the week after the debate.