I think you should maybe check the statistics on that.
It's simple. The math behind elections in first past the pole single member districts pretty much dictates no more than two effect poles to which all factions with a political interest are attracted. and around which they divide.
Groups of factions cluster around those two poles. Currently we call one Republican and the other Democrat.
Candidates are belched up from within the two groups after negotiations (or even primaries) between and among all the constituent factions.
That's where the candidates come from. Whether or not they get financing is pretty much decided by the factions ~ and that can be a real problem from time to time.
In all cases a winning candidate must be attractive to the Democrat or Republican "base:, as well as at least one faction in the other party or "pole".
The people who do not participate in this process are OUT OF THE GAME. They may vote in primaries but if they chose to run a candidate outside of the party main stream the candidate will undoubtedly lose due to lack of financial backing (unless he's really rich).
Independents do not count in this process. They don't really count in the general elections either ~ they are a thin fringe. There are, however, many Democrats who claim they are INDEPENDENTS because that gets them off the mailing lists and the telephone calls. Republicans can be found who will do this too, but to a lesser degree.
Effectively, when you count the Democrats with the Independents you come up with more than there are Republicans. However, Republicans continue to win elections on a broad basis by appealing to their base AND to different factions within the Democrat base (depending on district and part of the country).
Political parties pay for internal polls ~ the results of which they never reveal. That polling is much more precise and meaningful than the public polls which are usually conducted by and on behalf of political interest groups.
Most of the polls matching up Obama and various Republican candidates are done for the purpose of manipulating Republican opinion.