It's certainly true that the candidate that raises more money tends to win elections. But correlation is not the same thing as causation. That is, just because the winner had more money does not mean that money was the deciding factor. Moreover the trend that the more well-financed candidate tends to win is not a hard-and-fast rule. The less well-funded candidate can win, and the current presidential primaries, especially on the Republican side, are a perfect example. Jeb Bush is the most well-financed candidate in the GOP and he appears to be just about on his last legs.
If anecdotal examples of the money underdog winning aren't enough, there's also the curious fact that candidates in districts with effectively no competition still receive donations. This suggests that the primary purpose of donors may be less about determining a winner and more about securing influence with whoever wins. Why else would you donate to someone who was all but guaranteed a victory?
Today, the need to get money out of politics is often taken as a given, by many Republicans and Democrats alike. The most prominent solutions involve some kind of campaign finance reform, which is supposed to even the playing field. But all of this is based on an assumption that may not hold--that money determines elections. And before we jump to a solution, we should always be sure to understand the problem.