Capitalist Democracy or Capitalism vs. Democracy?
A few things happen in the runup to an election in a capitalist democracy.
First, frontrunners are determined. Frontrunners are defined by the media as those who have raised the most money - in other words, those with more rich backers are more likely to be frontrunners.
The second thing that happens is deciding who makes it into the initial debates broadcast by the media. The media basically decides that also by who has raised the most money. So those with more rich backers are more likely to be included in debates (and considered "non-fringe" choices).
Spending the Campaign Contributions
The third thing that happens is, of course, the money raised is actually spent: attack ads, infomercials, getting signatures to make it on to ballots, telephone ads, rally organizing, buttons, T-shirts, and let's not forget the "all-important" bumper stickers - all that movement building costs money, and once again, those with more rich backers (or those who can pump in a lot of their own money) are more likely to be able to build their following.
Coverage of Candidates
Finally, of course, is ownership of the mass media itself. Obviously the mass media is controlled by a few very wealthy people - will they sit idly by and give "fair and balanced" reporting to a candidate that has different politics than they do? I doubt it. What is more likely to happen is that the more their politics differ, the more hostile the reporting will be. It would be quite interesting if media outlets opened up their editorial and coverage decision-making process to the voting public.
Control the Ideas, Control the Vote
Not only does control of the media allow controlling which ideas get disseminated, wealth also allows for the "production" of more rationalizations for your ideas, by funding think tanks and hiring armies of lawyers and lobbyists. If an elected official does not turn out to act like you had hoped, they can be weeded out by withholding or redirecting campaign contributions, or "brought back into the fold" by threatening their political positions with media and think tank power.
Considering how often capitalism and democracy are at odds, the interesting question to ask for those who believe they are living in a "successful capitalist democracy" is: How much of your country's success do you think is due to capitalism and how much is due to democracy? ...but that would have to be a topic for a different node.
See also http://www.pbs.org/wgbh/pages/frontline/president/note.html