The Republican committee that handles Senate campaigns picked up the pace in fund raising in February, but still trails its rival Democratic committee by almost a 2-1 margin.
Democrats still hold a big advantage on money in the bank with $27.4 million, while Republicans have $14.5 million.
"We had a huge January and February. Where we're seeing progress is new money through small donors in the direct mail program," said Brian Nick, an NRSC spokesman.
The GOP now holds a 55-44 advantage in the Senate, with one Democratic-leaning independent.
DSCC spokesman Phil Singer said: "Senate Democrats have twice as much money in the bank as the GOP and have several candidates who are already leading their Republican opponents in the polls. We're exactly where we want to be at this stage."
GOP strategists have said the Republican National Committee will have enough money to help if needed.
The RNC raised $10.8 million in February and has $40.8 million in the bank, while the DNC raised $6 million and has $8.9 million on hand.
The parties' congressional campaign committees are running fairly close to each other in fund raising.