Former U.S. Attorney General Eric Holder warned 2020 Democratic candidates to "be wary of attacking" former President Barack Obama's record.

Holder's message came Wednesday following the second Democratic debate in Detroit. His warning came after Joe Biden, the 2020 presidential front-runner, was forced to answer for a number of policies from his time as vice president under Obama.

"To my fellow Democrats, " the Obama-era A.G. tweeted. "Be wary of attacking the Obama record. Build on it. Expand it. But there is little to be gained - for you or the party - by attacking a very successful and still popular Democratic President."

To my fellow Democrats. Be wary of attacking the Obama record. Build on it. Expand it. But there is little to be gained - for you or the party - by attacking a very successful and still popular Democratic President. — Eric Holder (@EricHolder) August 1, 2019

At one point in the debate, Biden was asked if his presidency would result in high numbers of deportations rivaling those of the Obama administration. Hecklers interrupted his response, screaming, "3 million deportations!" in reference to the number of people deported under the last administration.

New York City Mayor Bill de Blasio also hit Biden for the deportations under Obama, asking him point blank if tried to stop them from happening.

At another tense moment in the debate, New Jersey Sen. Cory Booker called out the former vice president for invoking Obama's legacy when it's convenient.

"Mr. Vice President, you can't have it both ways. You invoke President Obama more than anybody in this campaign. You can't do it when it is convenient and dodge it when it is not," Booker said.