2018 VOTER GUIDE: A look at all the California propositions
The two Democrats faced off at the non-partisan Public Policy Institute of California in San Francisco. The group's president, Mark Baldassare, will moderate the debate.
The debate started at noon.
2018 VOTER GUIDE: A look at California's gubernatorial candidates
De Leon received the California Democratic Party's nomination in July, but Feinstein leads in public polling.
She also has about $4 million left in her campaign to spend compared to de Leon's less than half a million. Feinstein is seeking a sixth term as senator.
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The Associated Press contributed to this report.