Former Vice President Joe Biden reached out to at least a half-dozen supporters Tuesday to tell them he intends to run for president and to ask for their help in lining up contributions from major donors so he can quickly raise several million dollars, said a person familiar with the matter.

Mr. Biden has expressed concern to these people that he wouldn’t be able to raise millions of dollars in online donations immediately the way some other Democratic candidates have, including former Texas Rep. Beto O’Rourke and Vermont Sen. Bernie Sanders, this person said. Mr. O’Rourke raised $6.1 million in the 24 hours after his announcement, while Mr. Sanders collected $5.9 million in the same period.

Mr. Biden wants to announce a large fundraising number after his candidacy is official to better compete in what is often dubbed the “money primary” that kicks off a presidential season.