The confrontation between Democratic Reps. Brad Sherman and Howard Berman came amid a nasty campaign for a Los Angeles-area seat.
The lunge by the 57-year-old Sherman appeared to startle Berman, who is 71 and smaller in stature, while the audience hooted and shouted.
Moments earlier during a dispute on an immigration-related bill Berman stepped close to Sherman on the stage, who told his colleague loudly, "Get out of my face."
Berman's campaign released a video of the confrontation at Pierce College in the San Fernando Valley.
The veteran lawmakers are running against each other because of California's new primary system, which sends the top two finishers to the general election regardless of party affiliation.
Sherman said in a statement that the debate "was not conducted at the highest level. I regret my part in allowing emotions distract from the exchange of views."