As Jason Furman explains here, and as Len Burman explains here, the best evidence suggests otherwise. Cutting these taxes may lead to a temporary spike in revenue, because people sell stock to realize gains while rates are low. But over the long term, the biggest owners of stock—the wealthiest Americans—will mostly save what they will save, regardless of fluctuations in the rate.
The Congressional Budget Office notes that “the potentially large difference between the long- and short-term sensitivity of realizations to tax rates can mislead observers into assuming a greater permanent responsiveness than actually exists.”
Salary in the highest tax bracket is taxed at 35 percent, but profits from stocks held long enough to be called "long-term capital gains" are taxed at 15 percent. A hedge fund manager making $100 million in a year would pay $15 million to the government if he is able to take his income as capital gains, not the $35 million he would have to pay if the income was considered salary.