Richard Carmona's haul for April through June was his most lucrative to date, and left him with about $1.6 million cash on hand, according to fundraising figures obtained by The Associated Press.
His campaign says 86 percent of donors gave $250 or less.
Carmona, a former U.S. surgeon general, is the only Democrat running. Arizona Rep. Jeff Flake and businessman Wil Cardon are among the Republicans competing in the primary.
Flake and Cardon haven't released their second-quarter totals. But Flake had more than $3 million in the bank at the end of March. Cardon has already loaned his campaign millions of dollars.
Cardon and Flake are both on the air with ads attacking each other.