The 81-year-old president and his senior advisers had expressed the concerns about whether the veep was up to the challenge of facing off against Trump in the lead up to his historic announcement, Axios reported Monday, citing three Biden aides familiar with the conversations.

Still, Biden ultimately ended up endorsing Harris, 59, when he revealed he was stepping aside Sunday — weeks after his train wreck debate against Trump threw his campaign into disarray.

Biden and Harris spoke repeatedly over the phone in the hours before the bombshell announcement was made public, sources said.

Harris, on her part, spent more than 10 hours making frantic calls to more than 100 Democratic party leaders, politicians and donors to rally support, one source told CNN.

Former Presidents Barack Obama and Bill Clinton were among the flurry of calls she placed, according to the source.

While Bill and Hillary Clinton wasted little time in publicly backing Harris, Obama didn’t immediately offer up his endorsement — saying Democrats would pick an unnamed “outstanding nominee.”

“We will be navigating uncharted waters in the days ahead,” Obama said. “But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.”

Just minutes after Biden endorsed Harris, the VP posted on X that she was “honored” to have his vote of confidence and her “intention is to earn and win this nomination.”

The Democratic National Committee is expected to host a virtual vote on the presidential nomination in early August to meet an Ohio ballot-access deadline on Aug. 7.