So, he wants to get rid of state and local tax deductions? Then you would be taxed double on the taxes you pay to the state and local government: You would have to pay taxes on your total income and then pay taxes again on the money you sent to the state, etc.
That's one of the reasons this is so unpopular.
He wants to get rid of the home mortgage interest deduction? But if you buy property to rent out, you get to deduct the mortgage interest as part of your business expenses. Buying a house is probably the single biggest investment people make. Why not let them deduct the investment expense (the mortgage interest). If you outlaw that then people just pair up and buy rental property and rent the houses to each other so they get the deduction. If you decide to not let people deduct mortgage interest on rentals as a business expense, then are you going to deny every business expense? Do that and you destroy the American economy.
