CNN host Jake Tapper Jacob (Jake) Paul TapperThe media's misleading use of COVID-19 data Julia Louis-Dreyfus: 'We can't spend much time grieving' Ginsburg Pence aide dismisses concerns rushed vote on Trump nominee will hurt vulnerable senators MORE pressed Rep. Seth Moulton Seth MoultonThe Hill's Morning Report - Presented by Facebook - Markey defeats Kennedy; Trump lauds America's enforcers in Wisconsin Moulton fends off primary challenges in Massachusetts Portland: The Pentagon should step up or pipe down MORE (D-Mass.), a 2020 White House hopeful, on Sunday on whether he thought then-Sen. Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE's vote for the Iraq War was good or not, calling Moulton's response a "cop out."

"You fought in Iraq even though intellectually you opposed the war. Joe Biden was in the Senate at the time, he voted to go to war in Iraq, was that a mistake?" Tapper asked during the network's "State of the Union," referring to the former vice president who is also seeking the Democratic presidential nomination.

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"I have a lot of respect for Joe Biden, he's a mentor and a friend, but I do think that it's time for the generation that fought in Iraq and Afghanistan to step in," Moulton initially responded.

"But was it a mistake for him to vote to go to war in Iraq?" Tapper asked again.

"Well, I wasn't in the Senate at the time so I'm not going to say that, but," Moulton began to answer before Tapper cut him off.

"I'm sorry, but that's a cop out. You were in the front lines, you have more of a right to make a judgement about that vote than anyone I've interviewed who is running for president," Tapper said.

"OK, fair," Moulton relented, "It was a mistake, because we should have been a lot more careful about going into Iraq, we should have questioned the intelligence."

Some Democrats have pointed to Biden's vote for the Iraq War in 2002 as a reason he may not be liberal enough to win the primary, given the party's leftward shift since then.

Biden is leading the Democratic field, averaging 35 percent support in recent polling, according to RealClearPolitics. Moulton averages 0.2 percent support.