WASHINGTON — As soon as the last major tax law was enacted in the fall of 1986, accountant Edward Mendlowitz remembers, he worked around the clock to convert corporations into partnerships and other so-called pass-through businesses to take advantage of the new tax code.

Now with congressional Republicans nearing passage of $1.5 trillion in net tax cuts, mostly for corporations, Mendlowitz reckons he may soon be doing the reverse. The corporate tax rate is expected to fall from 35 percent to about 20 percent, its lowest in more than 75 years and considerably less than what pass-through taxpayers are likely to face.

"We're not taking the pencil and paper out yet, but at some point we're probably going to switch a lot of them," said Mendlowitz, a partner with WithumSmith Brown in New Brunswick, N.J.

The specter of mass conversions — and the potentially huge tax revenue losses resulting from them — is but one vexing problem for Republican leaders as they try to stay within debt limits and resolve differences in how the Senate and House bills would treat pass-through entities, which account for about 95 percent of all businesses.

In the view of tax experts, the two Republican versions on taxing pass-throughs are far apart. Where the House and Senate come down on the matter could have implications not only for the federal debt, but also the level of entrepreneurial activity and even employer-employee relationships, not to mention whether GOP leaders ultimately succeed in passing final tax legislation.

Analysts also see the pass-through provisions as particularly susceptible to workarounds and manipulation. Despite anti-abuse rules that are included in the bills, the haste and unconventional manner in which Republican leaders are trying to push through their tax plan adds to the risk of leaving loopholes that clever lawyers and accountants will almost certainly be able to exploit.

For example, the proposed Senate changes could spur some salaried employees to make an arrangement with their employers to form their own companies to take advantage of the pass-through deductions not allowed for individual taxpayers. That could further increase the already growing trend of consultants and independent contractors.

"Say you've got two businesses in the same industry," said Howard Wagner, managing director at Crowe Horwath LLP, a public accounting firm. "One business model is to hire employees, the other is to use independent contractors. Will the one who is able to use independent contractors be able to attract better talent if the lower rate is available to those individuals?"

"These things have real-world consequences," he said.

The bills now in conference in the Senate and the House, both under Republican control, share key provisions. They are fundamentally aimed at making big corporate tax cuts permanent and shifting to a territorial system in which only domestically generated income would be taxed.

The Trump administration and Republican backers in Congress argue that the cuts will increase investments, stimulate economic growth, and lead to sizable wage gains for the average American.

Congressional Democrats and many economists doubt the tax savings to already cash-rich corporations and individuals would trickle down to middle-class households, and they worry that the huge tax cuts would further swell the nation's public debt, triggering higher interest rates that could neutralize potential economic gains from changing the tax code.

The House tax bill passed in mid-November, the Senate's Dec. 2. There are important, clear-cut differences, notably over whether to repeal the individual mandate in the Affordable Care Act and how to handle the alternative minimum tax.

But the pass-through provisions may be the most complicated and fraught with uncertainties in terms of how they will play out and the potential cost to Uncle Sam.

Pass-through entities are so named because their incomes, for federal tax purposes, are treated as having "passed through" directly to the owners. Most pass-throughs are mom-and-pop operators, but they include large law firms and hedge funds that are structured as partnerships or what are known as S corporations, as opposed to C corporations like publicly traded Wal-Mart and Microsoft. Only C corporations are subject to the federal corporate income tax, now at 35 percent.

Pass-through owners are currently taxed at the same rates for individuals, with the top tax bracket at 39.6 percent.

The Senate tax plan would slightly lower the maximum individual tax rate to 38.5 percent, and would provide a special 23 percent deduction on qualifying pass-through income. For pass-through owners in the highest tax bracket, that means their effective tax rate would be a little less than 30 percent.

The House measure takes a different approach. It would keep the 39.6 percent marginal tax rate for individuals, but sets a new top rate of 25 percent for pass-throughs. Owners of professional services such as legal and consulting businesses would not qualify — although real estate investment firms like those controlled by President Trump would — while the Senate version would allow those and many other service providers to take the pass-through deduction from their incomes as long as they do not exceed $500,000 for couples.

The lower House rate for pass-throughs — 25 percent compared with the effective rate of 30 percent with the Senate bill — means it would be more expensive to go with the House plan. The cost of the pass-through tax cuts envisioned by the House is about $600 billion over 10 years, while the congressional Joint Committee on Taxation estimates it at $340 billion for the Senate plan.

That is a big gap that will be hard to bridge, but it is further complicated by the fact that both Republican versions for treating pass-throughs could have major unintended but foreseeable consequences.

On the Senate side, even with the special 23 percent deduction, the effective tax rate for high-profit pass-throughs would be almost 10 percentage points higher than the 20 percent marginal rate for C corporations, and that does not include the deduction that corporations can take for state and local taxes. (Those deductions would be eliminated for taxpayers filing individual returns.)