Romney's tax plan has never been spelled out completely. It starts with a 20% across the board cut for everyone. That is where Obama gets the $5 trillion revenue loss figure, but Romney says he will make the cuts revenue neutral by eliminating deductions, refusing to specify which.
That's where the problem lies. Some taxpayers have a lot of deductions and might see an increase in their taxes, while others with few deductions would get a large tax cut. It might average out, but you don't want to be on the wrong side of that averaging.
What's missing from the discussion is that any tax plan will have to be passed by Congress. Many deductions are aimed at certain businesses. The housing and real estate lobbies will argue fiercely against eliminating the deduction for home mortgage interest and that is a big one—for many taxpayers, their largest deduction. State and local income taxes are a big deduction in some states, but not in others. Where income tax is high the Congressional delegations from those states will vote against eliminating the deduction and Romney may not get the support of his party on that one.
Voters may well go for the 20% income tax cut, not realizing what they could lose in deductions. As long as Romney refuses to tell, he may win votes. The clincher is that the Tax Policy Center claims that no combination of eliminated deductions adds up to enough to allow a 20% tax cut. Analysts who disagree rely on a large increase in economic growth to offset the shortfall.