That the Clinton campaign is worried about being 'outraised' by Sanders -- despite that Sanders has a maximum donation limit of $2700 per donor -- says a lot about the growing support for Sanders. While it certainly seems possible at this point in the election that Sanders could raise more money for his campaign than Clinton, but whether or not he does, in fact, 'outraise' her will not be the key factor in determining his win over Clinton. What has largely garnered support for Sanders over the last few months has been his lack of interest in raising campaign money and his refusal to accept large donations from corporations -- unlike any of his competitors, including Clinton. In the end, it will likely be in spite of having fewer funds that Sanders will win the election.
Sanders has done well with individual donors which has certainly boosted his chances. Voters seem to be looking for candidates that are not from the established areas of the parties. Clinton has had her own problems with the email and Benghazi affairs. Sanders does face an uphill battle as the democratic party has done what it can to help Clinton such as limiting the amount of debates.
The American population is savvy enough to realize that because one candidate is out-advertising another does not mean that they should be nominated for the presidency. Other factors must enter into the equation such as policy statements, credibility and likeliness to be elected. Clinton is a very strong candidate and to overtake her will require a strong candidate, not simply one that can outspend her.
Fundraising numbers, particularly in a primary contest, are not necessarily a prediction of the candidate's success at many crucial election tasks other than establishing base support. Fundraising provides no insight into the candidate's broad appeal to party moderates and independents and cannot accurately predict success in the general election. The dollars do help paint a picture of a candidate with many supporters, resources, and a serious chance at winning.