News that Republican lawmakers had delayed the release of their tax bill by one day served to underscore the enormous complexity of paying for tax cuts that by some estimates will add more than $2 trillion to the federal debt.

Both chambers of Congress have passed budget resolutions that will pave the way for $1.5 trillion in new deficit spending.

But that will just begin to cover the cost of lowering the corporate tax rate from 35 percent to 20 percent, and lowering individual bracket rates to zero, 10 percent, 25 percent, and 35 percent. Tax writers are reportedly considering keeping the existing top individual rate of 39.5 percent from income over $1 million.

New limits on the deductibility of deferrals to 401(k) and other defined contribution plans can be expected. Whether a new cap will be as low as the proposed $2,400 recently leaked to media outlets is another question.

Previous Republican tax proposals would have capped pre-tax contributions to 401(k)s at about $8,000, or half the existing statutory limit.

Representatives from the House Ways and Means Committee recently confirmed to BenefitsPRO that several retirement and savings policy changes are being considered.

One proposal, Universal Savings Accounts, would allow all Americans to invest after-tax dollars, up to an annual limit, that could grow tax-free and be withdrawn at any time, for any purpose, without penalty.

The devil, of course, will be in the details.

Here’s a list of 10 key factors that will invariably drive the tax discussion as the Ways and Means Committee marks up the tax reform bill next week:

1. More than $1 trillion

That’s how much the Treasury Department says traditional defined contribution plans like 401(k)s will cost in so-called tax expenditures between 2018 and 2027, the ten-year budget window through which the tax reform bill will be scored.

2. A top expenditure

While not the largest expenditure, workplace savings plans make the top five, according to Treasury.

Employer-sponsored health benefit plans are the most expensive to the feds, projected to be more than a $3 trillion expenditure over the next decade.

Removing deductions on state and local taxes has emerged as controversial of a proposal as the caps on 401(k) deductions, and nearly killed efforts to pass a budget resolution in the House. Republicans from New York, California, and New Jersey are holding fast on protecting the SALT deduction, which Treasury says amounts to a $1 trillion expenditure over the next decade.

3. IRAs

Discussion on tax reform and retirement has focused on 401(k) deferral limits. But changes to traditional IRAs may emerge. The Treasury Department says pre-tax contributions to IRAs account for a $256 billion expenditure over the relative budget window.

4. HSAs and other medical accounts

The tax incentives on Heath Savings Accounts have escaped attention in the run up to tax reform. Republicans favor the incentives when it comes to health care reform, but will they be tapped to pay for lower individual rates? Treasury says they account for a $142 billion expenditure.

5. Doubling standard deduction

The White House has said it wants to double the standard deduction that low and moderate-income individuals and families can claim as way to assure they benefit from tax reform.

In 2013, the IRS says that nearly 70 percent of Americans chose to take the standard deduction, as opposed to itemizing deductions like contributions to 401(k) plans.

As household income ticks up, so do the number of those that itemize. For households making $75,000 to $100,000, 59 percent itemized in 2013. Nearly 80 percent making between $100,000 and $200,000 itemized in 2013, as did 94 percent of households claiming more than $200,000 in income.

6. Less than 30 percent

Treasury says that less than 30 percent of households have access to a 401(k) or defined contribution plan. Low access is commonly cited by critics of 401(k) plans.

7. Will tax writers cap traditional 401(k) deferrals on higher earners only?

Critics also note that the overall benefits and tax preferences on traditional 401(k)s is reserved for higher earners.

Treasury says that 30 percent of the share of present value of the tax benefits on DC plans goes to households with income between $165,000 and $1.7 million.

That begs an obvious question: Will Republicans walk back comprehensive limits on traditional 401(k) contributions and instead place new caps only on higher earners?

8. Safe bet

For whatever emerges from the first draft of a tax bill, the safe money is that the leaked proposal of a $2,400 cap on traditional 401(k)s won’t survive the mark up process, if indeed Republicans try to go that low in the first draft.

Data from the Employee Benefits Research Institute shows that even lower-wage earners that contribute to 401(k) plans would be impacted, potentially disincentivizing them to save for retirement.

Nearly 40 percent of workers making between $10,000 and $25,000 defer more than $2,400. And 60 percent of workers making between $50,000 and $75,000 contribute more than $2,400.

9. Few in favor of capping 401(k)s

Opponents of tapping 401(k) deferrals to fund lower tax rates far outnumber proponents.

Republican lawmakers in the House and Senate have voiced concerns that reducing tax incentives to save for retirement would result in lower savings rates.

Then there is the “budget gimmick” claim, perhaps the thorniest complaint tax writers will have to fend from fiscal hawks in their own party.

The gimmick of course would be mandating some level of after-tax, or Roth contributions, in order to move tax receipts into the 10-year budget window to offset revenue losses from tax cuts.

But that raises implications for the federal budget outside the 10-year window, when much of the $5 trillion in 401(k)s will be taxed as retirement income.

Republicans intend to pass tax reform under budget reconciliation rules, which will require only a simple majority in the Senate. But those same rules require that the bill not raise budget deficits outside the 10-year budget window.

10. Progressives counter with more tax breaks

Experts in favor of making the tax incentives on 401(k)s more progressive—or shifting the benefits to lower-wage earners from higher-wage earners—have been quiet on the question of Rothification.

Democrats in Congress have used proposed caps on 401(k)s to foment claims that Republican tax reform would amount to a massive give away for the wealthy at the expense of the middle-class.

Those claims will undoubtedly continue to be aired in the coming weeks.

In fact, not only do Congressional Democrats not want to see limits lowered, they actually want to raise them. New Democratic-sponsored legislation would raise the current deferral limit of $18,000 to $24,500.