From the Wall Street Journal:

Digital advertising and fundraising capabilities, along with the increasing competition between parties, are among factors fueling spending, said Kyle Roberts, the chief executive of media-measurement firm [AdImpact].

“Fundraising on the internet is bigger than ever, and candidates have more reach into low dollars than ever before, and it results in more cash in the system,” he said. Election campaigns increasingly are able to amass considerable sums from small donations made online by supporters.

Read the full article here.