Joe Biden became visibly irate with CBS reporter Ed O'Keefe after the newsman asked the former vice president why he attacked Sen. Bernie Sanders this week.

"Yesterday, you said you accepted Bernie's apology. Now, you're attacking him," O'Keefe said. "Why are you doing that? Why wasn't his apology enough, Mr. Vice President? Why attack Sanders?"

"Why, why, why, why, why, why, why?" Biden asked. "You're getting nervous, man. Calm down, it's okay. He apologized for saying that I was corrupt. He didn't say anything about whether or not I was telling the truth about Social Security."

Biden's frustration comes as a CNN poll found Sanders leading the 2020 Democratic presidential field. Biden had been leading the pack for much of the early primary season but has seen his front-runner status fade as supporters of Sen. Elizabeth Warren have drifted toward Sanders's candidacy.

Biden, 77, and his son, Hunter Biden, are at the center of President Trump's impeachment. Republicans have called for both to testify on benefits that the younger Biden may have received from the Ukrainian gas company Burisma Holdings while his father was vice president.

[Read: Democratic pensioners fight it out as Sanders accuses Biden of slashing Social Security]