My Fed tax due increased by more than 15% of long term capital gains. Why is that?

Any chance you are using the CD/download product? If so, you can go into Forms mode and look for the Qualified Dividends and Capital Gains Worksheet (abbreviated as Qual Div/Cap Gn) and see how the tax is calculated.

If you are using the Online product, you will need to pay for your return in order to see this worksheet.

 Net Investment Income Tax 3.8% on top of Long Term Cap Gains.  Didn't know about it.