Want to know why the individual tax rate cuts in the Senate version of the Tax Cuts and Jobs Act are so small. Or conversely, why the bill would lose $1.5 trillion over 10 years even though all the individual income tax provisions are repealed by 2026?  Simple:  What began as a lower-the-rates, broaden-the-base tax reform seems to be leaving out much of the base-broadening.

Here is a list of the 10 largest individual tax expenditures, according to the Treasury Department:

Exclusion of employer contributions to health insurance, preferential rate for capital gains, defined contribution retirement plans, deductibility of state and local income taxes, defined benefit retirement plans, deduction for mortgage interest, deduction for charitable contributions, capital gains exclusion for home sales, plus the earned income and child credits (I left out the exclusion of net imputed rental income because only tax economists understand it).

Here is the list of the major tax expenditures the Senate Finance version of the Tax Cuts and Jobs Act would repeal: The state and local (SALT) tax deduction.

That’s it.

Indeed, most of the top 10 tax expenditures (they collectively reduce federal revenue by roughly $1 trillion-a-year) would be entirely untouched—except the child credit that would be expanded. Otherwise, a nip here and a tuck there. But except for SALT, essentially nothing.

The House bill targets a few mid-sized preferences such as repealing the deduction for medical expenses and makes a modest effort to trim the mortgage interest deduction. But the Senate Finance Committee measure doesn’t even touch these.  

It does go after the deductibility of casualty losses and employee moving expenses and the tax exclusion for bicycle commuter benefits. But these are loose change in the fiscal sofa cushions.

Why are the biggest tax preferences so well protected in the Senate bill? We don’t know for sure, of course. But I suspect the reason is simply politics. Once Republicans decided to go it alone and pass a tax bill without Democratic votes, they found themselves in a box. More ambitious tax rate cuts would have required much broader elimination of high-cost—and exceedingly popular—tax preferences. That’s what the bill proposed by former House Ways & Means Committee Dave Camp would have done. Even the House GOP leadership’s 2016 Better Way blueprint headed in that direction.

But without bipartisan support, Republicans just couldn’t go there. You can hear the withering criticism the GOP is getting over the Senate proposal to eliminate the SALT deduction. Imagine if the Senate threw mortgage interest, charitable giving, retirement savings, or employer-sponsored health insurance to the mix. It would be a political disaster. Democrats—and many voters-- would attack them mercilessly for eliminating popular tax breaks.

However, the consequences of preserving all these tax breaks are significant.

Compare the Senate Finance bill with the tax reform proposed by former House Ways & Means Committee chair Dave Camp (R-MI) in 2014. He would have brought the top individual income tax rate down to 35 percent and increased the Earned Income Tax Credit. And he did it without making major changes in people’s average after-tax income, no matter what their income level.  

The Finance Committee could get the top rate down from 39.6 percent to only 38.5 percent and it has ignored the EITC. But to get there and not lose revenue (at least not inside the 10-year budget window), Camp had to eliminate or scale back a long list of popular tax breaks including those for mortgage interest, charitable giving, and retirement savings—as well as SALT.

 You may remember the response from then-House Speaker John Boehner:  “Blah, blah, blah,” the Speaker said. Camp’s reform went nowhere and he retired soon after proposing the bill.

The Senate leadership made a choice to not take the same road Camp did. The result: A bill that looks much more like a tax cut for high-income households than tax reform.