Not by elite donors though. As I understand, it was donations from a very large user base: https://en.wikipedia.org/wiki/Barack_Obama_presidential_camp...

The campaign raised much of its cash in small donations over the internet, with about half of its intake coming in increments of less than $200.[53] Both major party campaigns screened regularly for patterns of abuse and returned or rejected donations in excess of legal limits, from overseas, from untraceable addresses, or from fraudulent names

and his second campaign had 3 times larger user base.

He was such an overwhelming favorite by the end which is what I believe fueled his donations. No point in funding a loser.

Also was the Big Data candidate: http://www.nytimes.com/2012/11/17/opinion/beware-the-big-dat...