Despite being outspent, Biden's campaign finished the month surging at the polls with a victory in the February 29 nomination contest in South Carolina.
Since early March, the former vice president has led the Democratic contest to be the nominee to take on Republican President Donald Trump in the November presidential election.
In February, Biden's campaign spend $13.1 million, compared to $45.8 million spent by Sanders. His campaign manager said this week he was reassessing his bid.