Democrats need a net gain of 23 seats to take control of the House in November, and fewer than 50 of the 435 districts are considered tossups by political pundits tracking the race. So far, they've outraised their GOP rivals in overall contributions in a little more than half of those races.

This midterm election is expected to set new overall records for campaign contributions, in part because of the recent rise in the amounts of "soft money" spending through political organizations called SuperPacs that allow unlimited contributions.

These SuperPACs have already begun raising sizable war chests for both parties. Much of this so-called "outside" spending is used to pay for TV and digital advertising, the bulk of which doesn't begin until closer to the election, after the primary season is over. (The CNBC analysis of battleground districts includes only direct contributions to individual candidates.)