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...