(CNN) – The Democratic Senatorial Campaign Committee brought in $5.6 million in May, while their GOP counterparts brought in $3.9 million, according to the fund-raising arms in charge of electing the parties' candidates to the upper chamber.
The DSCC ended the month with $28.3 million cash on hand and the National Republican Senatorial Committee had $23 million available, the groups said Wednesday.
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Numbers released by the groups show the Democrats outraising the Republicans this cycle, $70 million to $64.4 million.
The DSCC credited their success in part to support from grassroots donors.
"Despite the millions being spent by secret groups around the country, Democrats are working hard to keep the Senate and our supporters are more enthusiastic than ever. Over 99 percent of our donations are coming from the grassroots and our online donations have more than doubled," DSCC Executive Director Guy Cecil said in a statement.
The DSCC outraised the NRSC for the last four months, prior to May, edging out their Republican counterparts by $250,000 in April.
Republicans need a net gain of four seats in the next election to take control of the Senate, where Democrats are currently in the majority.