Some donors are wondering if Biden, age 81, will be able to withstand the tough conditions of the presidential campaign, according to the report.
Donors are also worried that Biden will face difficulties if he engages in direct debates with his rival, Donald Trump, who is four years younger, the report said.
However, Biden's advisers insisted that he only uses notecards for very detailed and technical questions. They also said that he frequently does spontaneous Q&A sessions, according to the report.
In many cases, Bided uses notecards because his detail-oriented staff want to ensure that he gives comprehensive answers and that his fundraisers are successful, the report added.