You know that a tax preference is in trouble when it begins to be labeled a "loophole" in the popular press. The tax freedom for municipal bond interest is starting to get that treatment. See this, for example.
Personally, I think that eliminating the tax freedom for all future muni bonds is a great idea. Of course, the value of existing tax-free bonds will then zoom, so there would be some windfall profits. That's the transition price to pay.