Are you familiar with the concept of optimal tax rates? Basically, if you tax something at 0% then you gain no revenue, but if you tax something at 100%, then no one will work/trade or otherwise obtain that income so, once again the government will take in no revenue.
With Income taxes, the I've read that the optimal tax rate is estimated to be as high as 50% depending on the person's income, while with regard to capital gains taxes the optimal revenue is gained at a much lower percentage. This is due to the fact that an investor holding stock A might prefer to own stock B, but he/she will continue to hold stock A rather than make a trade, due to tax implications. This hurts both the individual, who doesn't own the stock that they would like, and the government by not gaining the additional revenue.
This example was outlined perfectly by Reagans tax cut - which significantly dropped the tax rates on all individuals, but instead of obtaining less income tax revenue, the government actually saw tax revenue increase. No one knows what the exact point of revenue maximization is, but these are the relative levels I've seen published...
I agree that the Mitt Romney issue could lead to higher cap. gains taxes, but I'm certainly hoping that it only bumps them up to 20% or so... anything higher would be very counter-productive.