“The stakes of this election could not be higher, and by Election Day, every battleground voter will know it,” said Chauncey McLean, the president of Future Forward’s super PAC. “We’ll run a cost-effective and data-driven program of unprecedented scale to re-elect Joe Biden.”

The $250 million ad purchase is more than any independent expenditure group spent for the whole of the 2020 election. Future Forward spent $151 million to back Mr. Biden four years ago and is set to play an even more significant role in 2024, after the White House designated the super PAC as the leading outside group this year. The leading super PAC backing President Donald J. Trump in 2020, America First Action, spent $150 million.

By reserving ads early, the super PAC is expected to be able to negotiate better rates than those that buy later, and to secure some of the most coveted time slots.

President Biden and his advisers are elevating a new outside group as the leading super PAC to help re-elect him in 2024, making it the top destination for large sums of cash from supportive billionaires and multimillionaires.

The blessing of the group, Future Forward, is a changing of the guard in the important world of big-money Democratic politics. Since the 2012 election, a different group, Priorities USA, has been the leading super PAC for Democratic presidential candidates.

“In 2020, when they really appeared from nowhere and started placing advertising, the Biden campaign was impressed by the effectiveness of the ads and the overall rigorous testing that had clearly gone into the entire project,” Anita Dunn, a senior White House adviser to Mr. Biden, said of Future Forward. The group, she added, had “really earned its place as the pre-eminent super PAC supporting the Biden-Harris agenda and 2024 efforts.”