One huge advantage is incumbency. Those who have won election in the past begin any race with the advantage of having already-built fundraising networks. On average, congressional incumbents in 2012 raised more than double the amount of money brought in by their challengers — and boasted a 90 percent reelection rate.
Also, some would argue that in many cases the candidates who win the most votes do so based on the same electability, popularity and qualifications that make them the best at fundraising, and vice versa. A candidate who is compelling enough to get you to open your wallet should, in theory, also be able to get you to head to the ballot box for him or her.