Sen. Mark Udall (D-Colo.) outraised his Republican challenger between April and June.
Udall's campaign said Monday that it raised $3.1 million between April and June. It ended June with more than $5.7 million in the bank.
Rep. Cory Gardner, Udall's GOP opponent, announced last week that he raised $2.7 million during the same period. He wrapped up June with more than $3.4 million in the bank.
Garnder and Udall are running in one of the biggest battleground contests of 2014. Polls show a close race.
Second quarter fundraising reports are due to the Federal Election Commission by Tuesday.