"He knows he can beat Trump," Jim Moran, a former Democratic House member from Virginia, told Insider.
As Trump trashed Biden while announcing his third presidential bid, Biden responded with a video detailing the low points of Trump's record on Twitter, writing "Donald Trump failed America."
"The obligation would be cataclysmic, existential, monumental," the friend told Insider in an August interview.
On Friday, the progressive "Don't Run Joe" campaign released a birthday greeting for Biden, urging him not to seek re-election.
"If our focus is on winning, then going with the guy who knows how to beat Donald Trump is a pretty smart play," Sen. Chris Murphy, a Connecticut Democrat, previously told Insider.