The answer is: not as much as you might think. While there are differences between types of tax expenditures, most of the largest tax expenditures help a few people in the middle class a bit while helping wealthier Americans much more. The tax code may have become a type of middle-class welfare state, but in its current form is one of the most regressive and least efficient middle-class welfare states possible.
However, with median income on the decline, certain tax expenditures do play a role for some members of the middle class even as the highest earners gain an outsized share of the benefits and the system is highly regressive overall.
Think of tax expenditures like serving a very fancy and expensive dinner to Joe (who is very wealthy) and John (who is middle class). We fill Joe's plate with food, including heaping servings of mashed potatoes, ribs, broccoli, and dessert. On John's plate, we put one small half-dollop of mashed potatoes and a few peas. John does benefit from the dinner overall, but barely, while Joe gets massive benefits.
And this fancy dinner costs a lot of money.
In 2011, tax expenditures cost about 1.1 trillion dollars. The Office of Management and Budget projects a similar figure for the current fiscal year (FY 2013), with costs increasing gradually in the upcoming years. The costs mainly include deductions (like the home mortgage interest deduction), exclusions (like the employer-sponsored insurance exclusion), credits (like the earned income tax credit), and lower rates for investment income.
Here is how much middle-income earners have benefited from some of the largest tax expenditures (and estimates of how much they will cost in FY 2013):
Home Mortgage Interest Deduction (100.9 billion dollars in FY 2013): Income filers making 100,000 dollars per year or more got nearly 75% of the benefit from the home mortgage interest deduction in total dollars in 2011. Therefore, filers making less than 100,000 dollars per year, or a vast majority of Americans, got 25% of the benefit.
If we define the middle class broadly as the three middle quintiles (20-80% of earners), averages of Tax Policy Center estimates show that about 76 percent of middle class individuals would not see any increase in their taxes in 2012 if the mortgage interest deduction were eliminated. The average amount that a person in this quintile benefits from the deduction is 312 dollars per year. In other words, for a program that costs on average 559 dollars per person, a person in the middle group would benefit an average of 312 dollars.
Looking at total dollar amounts shows how benefits are distributed overall and is not affected by huge income disparities. To determine how these tax expenditures affect families' budgets, however, we can also look at benefits as a percentage of income. If we use 100,000 dollars per year in cash income as a cutoff for the middle class (which corresponds approximately to the cutoff for the 80th percentile of earners), we can see from the chart below that tax filers in the higher end of that spectrum - and especially right above it - benefited in the realm of 1% of their income on average from the home mortgage interest deduction in 2011.