Bennet's campaign reported Monday the senator, who is seeking re-election, raised nearly $1.8 million in the first quarter of 2016.
Five Republicans seeking to challenge Bennet raised lesser totals.
Colorado Springs businessman Robert Blaha's campaign reported $670,000 cash on hand as of March 31 after raising $1.1 million. Blaha donated $1 million of that to his own campaign.
Former state Rep. Jon Keyser had $200,000 in the bank after raising $300,000.
Also running are El Paso County Commissioner Darryl Glenn, former Aurora city councilman Ryan Frazier and former Colorado State University athletic director Jack Graham.
The GOP primary is June 28.