President Biden partly delayed dropping his 2024 re-election campaign because he doubted whether Vice President Kamala Harris could actually take on Donald Trump, a new report says.

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, for 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.

What to know about President Biden's decision to drop out of the 2024 presidential race:

• President Biden announced Sunday he is dropping out of the 2024 presidential race — after weeks of prominent Democrats and donors calling on him to withdraw following his disastrous performance in the first presidential debate.
• In a letter posted on X Sunday afternoon, Biden admitted that it is in the “best interest of my party and country” for him to step down as the Democratic nominee.
• Biden wrote that he intends to serve out the remainder of his term and will address the country on his decision later this week.
• In a follow-up X post, Biden endorsed Vice President Kamala Harris for the suddenly open Democratic nomination. “Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year,” the president wrote.
• Former President Donald Trump reacted to the news by labeling Biden the “worst president in the history of our country” in a call with CNN.

While Bill and Hillary Clinton wasted little time in publicly backing Harris, Obama didn’t immediately offer 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.