According The Washington Post, former President Obama had a private meeting with President Biden this summer where he warned him that former President Trump is a "more formidable candidate" than many Democrats realize.
After speaking with two people familiar with the meeting, Washington Post White House reporter Tyler Pager wrote about the private lunch Obama had with Biden in June during which the former president told Biden not to underestimate Trump and pledged to support Biden’s re-election bid.
According to the sources, Obama "voiced concern about Donald Trump’s political strengths — including an intensely loyal following, a Trump-friendly conservative media ecosystem and a polarized country — underlining his worry that Trump could be a more formidable candidate than many Democrats realize."
As such, the 44th president "promised to do all he could to help the president get reelected," which, Pager added, "was a welcome gesture for the White House at a time when Biden is eager to lock down promises of help from top Democrats."