Both Democrats and Republicans have considered phasing out the mortgage interest deduction, and there are good economic arguments for doing that. But it might depress an already weak housing market and hit some middle-class homeowners hard.

Eliminating the charitable deduction could devastate many philanthropic organizations and the people they serve. You can go down the line with many exemptions, deductions and credits and find an unintended, and unfortunate, consequence.

Likewise, more sweeping attempts to broaden the base can end up doing more harm than good.

Most states tax only retail sales to consumers — but some, for example, also tax sales to other businesses. This tax, called a gross receipts tax, certainly has a larger base than a retail sales tax since businesses at each stage of manufacturing, distribution and marketing end up being taxed.

Supporters of the idea say this cascading tax can be assessed at a much lower rate and still collect the same revenue over all, spreading out the pain. But it is a poorly designed tax, because it taxes products that involve many stages of production more than those produced in only one or two steps. That, in turn, encourages companies to merge to avoid paying multiple layers of tax — whether or not that makes any business sense.

To be sure, there are constructive ways to broaden the base. There are few compelling reasons, for example, that employer-provided health insurance, which is part of compensation, should be exempt from income tax. This tax break costs around $250 billion a year and makes gold-plated health insurance more attractive to workers, which drives up health costs. Eliminating it would be a good first step in shoring up the federal budget.

We could also turn the mortgage interest deduction into a flat 15 percent tax credit and cut the maximum deductible mortgage to $500,000, which would help many homeowners who do not itemize deductions, while curtailing subsidies for high-income people who don’t need help. This would raise about $40 billion in 2014, according to the nonpartisan Tax Policy Center. And we could allow a deduction for charitable contributions over 2 percent of adjusted gross income, which would save $20 billion in 2014 without discouraging most donations.