The Democratic Senatorial Campaign Committee outraised the National Republican Senatorial Committee for the fifth straight month in May, bringing in $5.6 million for the month to the NRSC's $3.9 million.

The DSCC also leads in cash on hand, with $28.3 in the bank to the NRSC's $23 million. But that edge will likely be negated by super-PAC spending, which Republican-affiliated groups are expected to dominate.

The two are fighting over a highly competitive Senate map: most observers expect the chamber to be narrowly divided after the next election.