When a person has investment capital, they already paid taxes on that money that is theirs.
There is a big difference between income and investments.
If you have a contract and will receive money for working, that is guaranteed income.
When you invest, some times you lose your investment. Because of that risk, the tax rate is lower.
If you raise the capital gains tax, you change the risk reward scenario.
People may decide to not invest if the taxes are raised. Think about it, investors who stop investing that do not have a job would owe zero taxes.
Mitt Romney is the perfect example. He could live off his money for the rest of his life and never work or invest again. In this example, his tax liability would be zero.
Rich folks could vote with their feet and leave the country and invest elsewhere taking their money with them.