Washington (CNN) Former Vice President Joe Biden said Thursday he was surprised by the "degree of the criticism" against former President Barack Obama at Wednesday's CNN Democratic debate.

Asked about attacks from 2020 rivals, Biden told reporters in Detroit, "I expected it."

But, he added, "I was a little surprised how much the incoming was about Barack."

Some Democratic candidates on Wednesday night raised Obama's immigration policies and the millions of deportations that occurred during his presidency as a way to challenge Biden, who has used his time as Obama's vice president as a centerpiece of his pitch to progressives. Biden fired back at candidates , questioning their record while defending his, despite his frequent promises early in his campaign to not speak ill of fellow Democrats.

On Thursday, Biden said the change in strategy was because he was provoked.

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