"If you run as a Democrat, you're a Democrat," he said. "Some Democrats are progressive, and some Democrats are not."
When he was asked if Ossoff was a progressive, Sanders then responded: "I don't know."
Ossoff backed Hillary Clinton over Sanders in last year's presidential primary, but has sought support from the progressive wing of the party as well as the more mainstream center.
Republicans have painted him as a creation of House Democratic leader Nancy Pelosi, an attack that Ossoff has criticized as a a tired smear from a predictable Republican playbook.