Politico:

So far, the latest figures show that the Democratic Party machinery has outraised its Republican counterpart in this campaign cycle by almost $270 million.
And even when outside spending on television advertising and direct mail is added to the mix, Republicans still haven’t closed the gap.
The money race totals come to $856 million for the Democratic committees and their aligned outside groups, compared to $677 for their Republican adversaries, based on figures compiled by the Center for Responsive Politics.
Included in that total: conservative groups have spent $169 million on ads attacking Democratic House and Senate candidates, compared to $80 million by liberal-leaning groups, based on figures as of Tuesday morning.

NYT:

Lost in all of the attention paid to the heavy spending by Republican-oriented independent groups in this year’s midterm elections is that Democratic candidates have generally wielded a significant head-to-head financial advantage over their Republican opponents in individual competitive races.
Even with a recent surge in fund-raising for Republican candidates, Democratic candidates have outraised their opponents over all by more than 30 percent in the 109 House races The New York Times has identified as in play. And Democratic candidates have significantly outspent their Republican counterparts over the last few months in those contests, $119 million to $79 million.