The biggest super PAC in American politics is in the middle of an unparalleled spending spree, unleashing more money on television advertising in the closing weeks of the 2024 race than the campaigns of Donald J. Trump and Kamala Harris combined.

The group, known as Future Forward, has ascended to the pinnacle of the Democratic political universe with remarkable speed, winning over some of the world’s richest people with grand promises of a “Moneyball” method to political advertising that it has pitched as the most sophisticated ever undertaken.

The group is, in some ways, an ad-making laboratory masquerading as a super PAC, testing thousands of messages, social media posts and ads in the 2024 race, ranking them in order of effectiveness and approving only those that resonate with voters. Ad makers produce roughly 20 potential commercials for every spot that ever airs. And Future Forward has conducted nearly four million voter surveys since Ms. Harris entered the race — and more than 10 million since January.

“They’re probably the most analytics- and evidence-driven PAC I’ve ever seen,” said David Nickerson, a political scientist who ran the experiments division of Barack Obama’s 2012 presidential campaign.

Publicly, Ms. Harris and Democratic leaders are appreciative of the group’s work. But Future Forward’s insular approach to spending the staggering $700 million it has raised in combination with its affiliated nonprofit group has led to suspicion and second-guessing, including inside Ms. Harris’s headquarters.

The story of Future Forward’s rise and its central if mostly hidden role in 2024 is based on interviews with more than four dozen Democratic strategists, donors and aides to Ms. Harris and the group, many of whom insisted on anonymity to share closely held information. The New York Times also reviewed internal Future Forward records and donor presentations.