In an interview this week with Time, Romney stated support for continuing deductions (or, if you prefer, loopholes) for mortgage interest, charitable donations and health insurance. As shown in the now-famous Tax Policy Center study (.pdf), it was already going to be pretty tough to design a revenue-neutral plan even when those expensive deductions were on the table. Putting them off limits makes any actual plan that much more unlikely to ever happen.
Romney will eventually be backed into a corner. There are only so many kinds of deductions whose elimination would raise enough money to make Romney's math work. Since Romney has passed on the chance to ax these deductions, he'll eventually have to announce his position on other costly tax expenditures like the child credit or the Earned Income Tax Credit.
It continues to amaze that he's able to give speeches across the country promising lower tax rates for everyone and receive cheers from audiences who don't concern themselves with how he might ever make it happen.