Robert Reich's 'Thoughts on Tax Day 2012' is worth noting if only for its reminder of two famous quotes.
Oliver Wendell Holmes, Jr.(1904): “taxes are the price we pay for a civilized society.”
Louis Brandeis (1897): “we may have a democracy or we may have great wealth concentrated in the hands of a few, but we cannot have both.”
One reason for believing the latter was attempted in a much more recent quote.
Abbott Joseph Liebling (1960): "Freedom of the press is guaranteed only to those who own one."
But this actually isn't quite right. The cost of publication was never all that high and now it is negligible. And so it's not the press but the audience that is hard or costly to obtain.
The reason Brandeis was right is because those with great wealth are better placed to buy or bribe the attention of voters to their message.
And the situation is unstable. Once a small class acquires more than half the wealth, that class has the power to buy more than half of the voters' attention and with the not unreasonable assumption of total gullibility (ie every viewer is immediately persuaded by the last message heard) that is enough for them to further cement their position. And even without total gullibility it may still be possible to persuade the majority with sufficiently repeated exposure so a high enough domination of the economy may be sufficient to control the politics so as to be self-perpetuating.