Each of the two main US parties has a base among the voters (though these seem to fading slowly). But the voters only get to vote once (or twice, if there's a contested primary). Campaign contributors vote continuously, before and after formal elections. After the election, the fact that one has made major financial contributions to the sitting President's campaign makes it more likely that he will listen to one's pleas (and of course I'm ignoring what he does while sitting). Give money to the candidate, to the Party, to organizations that sponsor "issue" ads, etc. and you get influence, roughly in proportion to the amount you spend. More than roughly in proportion: US politics seems to work following the principle of increasing returns, with the amount of bang one gets per buck rising with the number of bux contributed.
So (given the winner-take-all structure) the "loyal bases" of the two political parties are given the choice between the lesser of two evils. Given the understandably pragmatic and short-sighted take that most people have on electoral politics (and the absense of any kind of grassroots labor or social-democratic party), the bases are encouraged to be loyal by the lesser of two evils choice they're always given. (Look, Goebbels is more moderate and pragmatic; he'll keep the extremist wing of the Nazi Party (Himmler, et al) in line. Consider how bad it would be if Himmler were to take charge! cf. Philip K. Dick, THE MAN IN THE HIGH CASTLE.) Of course, this loyalty simply encourages the politicos to take the loyal bases for granted. They only get considered if they threaten to be less loyal (to stop voting, to support the other party, to join a "third" party).
Now obviously, just because one is rich doesn't get one elected: look at that twit Michael Huffington, who ran for Senator of California a few years ago. But his opponent had to raise megabux to counteract his efforts, biasing the whole game in favor of big money. As I said before, to some extent campaigning by the Marlboro Man cancels out that of the Camel Woman, but in the end the combined effect is convince people to smoke (vote for a candidate who really represents big money). Now the organized grassroots power of labor and other out groups (that don't rely on the "one dollar one vote" principle) can cancel out the effects of big money, but the lesser-of-two-evils people usually don't want to talk about this.
Jim Devine jdevine at popmail.lmu.edu & http://clawww.lmu.edu/Departments/ECON/jdevine.html