"There was a lot to learn and it was a new experience for all of us," Clinton said. "There was so much going on under the surface on social media, and I don't think we did a good enough job being aware of and understanding the impact that such assertions and falsehoods could have."

She continued: "Try to get your message beyond the sort of reality show environment that [Trump] is trying to create so that the American people have an idea about why they would vote for you, not just against Donald Trump."

Clinton also warned Biden that debating Trump is unlike any other experience she's had, and she advised him to get ready for punches out of left field.

"Be prepared on that debate stage to be standing there with somebody unlike anyone you've been involved with before in politics, someone who lies with impunity, who literally will say anything, try to throw you off your game," she said

Clinton said she and Biden are old acquaintances from their time in the Senate and from working together in the Obama administration. She said they have been in conversation throughout the campaign and that she was ready to help Biden and Kamala Harris, his running mate, through the election.

Clinton noted the uncertainty around the election, particularly as the White House campaigns against mail-in voting during the coronavirus pandemic and mail performance decreases amid changes introduced by GOP donor and Postmaster General Louis DeJoy. She added that it's "going to be a challenging campaign" facing an incumbent who "will say or do anything."

She echoed remarks she made at this year's Democratic National Convention, including a brief nod to the circumstances of her electoral defeat in 2016, which was in spite of her winning the popular vote.

"They could win 3 million more votes, but not be on their way to the White House," Clinton said.

Clinton also warned that foreign adversaries would intervene to help Trump in his election, as U.S. intelligence found occurred in 2016 and is continuing today. Trump has repeatedly denounced claims of foreign interference as a "witch hunt."

Only hours before Clinton's MSNBC interview, Biden formally accepted the Democratic Party's nomination Thursday night to face Trump in November. Throughout the convention, the party portrayed Biden as a national unifier and symbol of decency rather than merely an embodiment of the party platform.

Clinton warned Biden to keep on that track and not to get distracted with whatever verbal assaults and incendiary remarks Trump may throw at him.

"Don't get thrown off your game," Clinton said.