The Midterm Digital Divide [Google Data Shows GOP Outspending Dems Online]

Disclaimer: I work for BPI, who published the post above.

Also, this report from Wesleyan University is related and shows the same general trends. I would caution citing the data for specific races here as it pretty noisy data that relies on some assumptions.

http://mediaproject.wesleyan.edu/releases/091318-digital/