The Democratic Senatorial Campaign Committee raised $5.6 million during the month of May and finished the month with $28.3 million in the bank.
The National Republican Senatorial Committee has not released its May fundraising sum yet. It ended April with $21.6 million in the bank.
Democrats currently hold a 53-47 advantage in the upper chamber. The party is defending 23 seats to the GOP's 10 in November.