Sanders raised 20 million in January 2016 while Clinton raised 15 million. It was not the case in June 2015 when Clinton outraised Sanders by a wide margin. Gradually her fund raising capacity dwindled in comparison to Sanders as can be seen in the Chart below.
In an article to Washingtonpost, Phil Bump showed a correlation between how much each of the candidates raised and the state of the competition. Clinton’s favorability rating was far ahead in June and so was the amount of fund she raised than Sanders’. Ever since then both her favorability and the amount of fund raised plummeted to the current state where there is tie in national favorability as reported in recent Quinnipiac University poll and even worse, Sanders outraised her in January 2016.
According to Phil Bump, Sanders started to beat Clinton in daily fundraising from late 2015 with some regularity and on 31 Dec 2015, he outraised her by $129,000. Small donations received by Sanders played a vital role in outraising Clinton as Sanders could easily go back to collect small donations which averaged $27, while Clinton had less space to do so.