Sen. Jeanne Shaheen (D-N.H.) outraised former Republican senator Scott Brown (Mass.) between April and June by about $800,000.
Shaheen's campaign announced Monday that it raised more than $2.8 million during the second quarter of the year. Brown's campaign said last week that he raised more than $2 million.
Brown is the frontrunner to win the Republican nomination against Shaheen. A recent poll showed him trailing her by 10 points.
Shaheen's campaign ended June with more than $5.1 million in the bank. Brown has not released his cash on hand total.
Second-quarter fundraising reports are due to the Federal Election Commission by July 15.