The entire population may not have a strong enough feeling towards a specific candidate or a specific issue in order for them to want to donate to a campaign. If both sides of an issue are putting out their own one sided story to the issue then everyone has an equal chance of hearing both sides. If you were to force people to spend money on campaigns then it would be essentially the public financing campaigns that may or may not have a chance of succeeding
The problem is that, as things stand, you're never going to get both sides of the story; politicians on the left and the right "hear" only the wealthy, and other large concentrations of power - those who "speak" the loudest - and follow policies that cater to them, not the population at large.
In relation to political finance, there are several countries that have considerable amounts of public financing, in one or more of the forms it takes - Germany, Japan, Sweden, Israel possibly with the highest levels. Is there any reason why the US would not be able to function in such a model?