Though Biden did not name Warren in his essay, it was apparent that he was referencing the Massachusetts senator who snarked that the former vice president was running in the wrong primary after he criticized the vague contours of how she would pay for her $20 trillion Medicare for All plan.

"So if Joe Biden doesn't like that, I'm just not sure where he's going," she said Friday. "Democrats are not gonna win by repeating Republican talking points. … If anyone wants to defend keeping those high profits for insurance companies and those high profits for drug companies, and not making the top 1 percent pay a fair share in taxes, and not making corporations pay a fair share in taxes, then I think they're running in the wrong presidential primary."

The barb prompted Biden to write the essay outlining all his accomplishments for the Democratic Party throughout his career.

And he did not back down when asked about his criticisms of Warren in an interview Wednesday morning.

“If you don’t agree with Elizabeth Warren, you must somehow be not a Democrat. You must somehow be corrupt. You must somehow not be as smart as she is,” he told Joe Madison on SiriusXM’s Urban View. “It’s just something we don’t do in our party. It’s not who we are.”

“She has things in her plan that are just not realistic, but if you question it, she says you don’t understand or you’re talking like a Republican,” he continued. “It’s just an elitist attitude that it’s either my way or the highway.”

This is not the first time the two candidates have butted heads. They exchanged barbs in the October debate after Biden’s claim that he was the only candidate with major legislative achievements. Warren pointed to her own work, including helping create the Consumer Financial Protection Bureau after the financial crisis.

“I convinced people to vote for it,” he insisted.

“I am deeply grateful to President Obama, who fought so hard to make sure that agency was passed into law.”