The Senate Majority PAC raised $151 million from July through September, setting a record for its third-quarter haul.
The super PAC, which supports Democratic Senate candidates, has now collected more than $300 million this election cycle.
Individual donors provided much of the total, with small-dollar contributions growing sharply over the past year.
Democratic Senate candidates have generally out-raised their Republican opponents in direct campaign funds.
Outside groups tied to former President Donald Trump have narrowed that advantage through heavy independent expenditures.
Those Trump-affiliated organizations have spent hundreds of millions on ads attacking Democratic incumbents in key states.
Senate control remains closely contested, with several races tightening as spending climbs in the final weeks.
The record fundraising reflects both donor urgency and the growing role of super PACs in deciding Senate majorities.
