President Trump wants to cut business taxes — a lot. Trump on Wednesday is expected to propose slashing the corporate income tax rate to 15 percent from 35 percent, a dramatic cut that Trump and his backers argue would spur business investment and drive economic growth, and that critics — and most experts in tax policy — say would represent a big tax cut for wealthy Americans and cost trillions of dollars.

The idea of cutting corporate tax rates isn’t new or particularly controversial. Experts on both sides of the aisle agree that the existing U.S. corporate tax system is broken. The theoretical tax rate, 35 percent, is the highest in the developed world. But because the system is riddled with loopholes, many businesses pay a fraction of that rate. Those tax breaks aren’t evenly distributed: Big, powerful industries, especially those with large overseas operations, tend to pay much less in taxes than primarily domestic ones. Small companies, which can’t afford armies of tax attorneys and lobbyists, often pay more, too. And quirks in the tax code mean that companies have an incentive to keep cash overseas, where it can’t be reinvested in the U.S. economy.

What sets Trump’s proposal apart is its size. President Barack Obama once proposed cutting the corporate tax rate to 28 percent. House Republicans, in their 2016 tax plan, would cut the rate to 20 percent. Trump’s proposal — which is consistent with the tax plan he released during the campaign — would go much further. What isn’t clear, however, is whether he considers 15 percent to be an opening bid in a negotiation or a final number he expects Congress to hit.

So what number is reasonable? That’s partly an ideological question; conservatives tend to favor lower tax rates on both companies and individuals, while liberals want higher taxes, especially on corporations and the wealthy. But it is also a practical question. How the tax plan is designed will help determine how big a cut is realistic. While it isn’t clear whether Trump’s announcement on Wednesday will provide the details necessary to evaluate his plan, here are three key questions that will help determine both its fiscal and political viability.

Question 1: What counts as a business?

In theory, individuals pay taxes at the individual rate, and businesses pay at the corporate rate. Simple enough. But most U.S. businesses don’t pay the corporate rate. That’s because most businesses aren’t actually set up as corporations — they’re limited liability companies or partnerships or other entities that pass their profits straight through to their owners, where the income is taxed at the individual rate.

Right now, this distinction doesn’t matter that much because the gap between the corporate and individual tax rates is small. Under Trump’s plan, however, the choice of business structure would suddenly matter a lot. Businesses that are set up as corporations, which tend to be big, national or multinational companies, would pay a considerably lower rate than businesses set up as partnerships, many of which are small or mid-size businesses. That hardly seems fair.

An alternative — one that Trump is apparently embracing — would be to let partnerships and similar businesses pay the corporate rate. But that’s potentially even more unfair. Because not all partnerships are struggling small businesses — many of them are big companies, especially law firms, real estate developers and other professional businesses. As a result, taxing partnerships as businesses would represent a huge tax cut for the rich, possibly including Trump himself. According to the Tax Policy Center — which leans left but produces analysis that is widely respected — nearly 80 percent of business income that is reported on individual income tax returns goes to households earning at least $200,000 a year; 60 percent goes to households earning $500,000 or more. And that’s right now — if business income is taxed at a much lower rate, there will be a strong incentive for the rich to find ways to classify their earnings as “business income” wherever possible.

There are possible compromises here. Republicans in Congress have discussed plans that would allow some but not all partnership income to be taxed at the lower rate. But the wider the gap between the business and individual rate, the tougher those negotiations will be.

Question 2: What counts as income?

The central idea of most proposals to reform the corporate tax is that if more income is taxable (because there are fewer loopholes), you can tax it at a lower rate. (Or, in tax jargon: “Lower the rates, broaden the base.”) The trick is deciding which loopholes to get rid of. Every deduction and credit in the tax code has a built-in base of supporters who will argue that their loophole is good economic policy.

Take one example: Right now, businesses can deduct interest expenses on their income taxes. Many economists view that as bad policy, in part because it encourages companies to borrow money instead of selling shares. (The House Republican plan would have eliminated the deductibility of interest in conjunction with other changes.) But the provision has staunch defenders, most notably the real estate industry, which routinely relies on debt to fund development and acquisitions and to improve financial returns. Trump’s own real estate business has at times taken on substantial debt. If that and other deductions stay in place, then cutting the tax rate will be much more expensive. Which brings us to…

Question 3: What about the deficit?

Republicans have previously insisted that any tax reform be “budget neutral,” meaning that it wouldn’t increase the federal deficit. That means that if they want to cut corporate income taxes, they’ll have to make up the gap through higher taxes (or spending cuts) elsewhere.

House Speaker Paul Ryan’s solution to that problem was a complex mechanism known as a border-adjusted tax, which would have raised billions of dollars in revenue for the government and helped to offset cuts elsewhere. The BAT ran into intense opposition from retailers and other powerful business interests, however, and its prospects seem to have faded. (Trump reportedly will not include a border-adjustment provision in his Wednesday announcement.)

Trump’s alternative solution appears to be: Forget about budget neutrality. The tax plan that Trump presented on the campaign trail (which included cuts to personal as well as business taxes) would have reduced federal revenue by between $4.4 trillion and $6.2 trillion over the next decade, depending on whose estimates you believe. Treasury Secretary Steven Mnuchin on Monday argued that Trump’s new plan would “pay for itself with economic growth.” But even most conservative economists don’t think tax cuts help the economy grow enough to offset cuts as big as the ones Trump is proposing.

Abandoning budget neutrality also poses another problem for Trump, however. Senate rules effectively mean Republicans will need some Democratic votes in order to pass a bill that increases the deficit over the long term. As a result, any tax cuts that aren’t paid for will likely have to be temporary. That’s a significant disadvantage for corporate tax reform in particular because businesses often plan years in advance and want to know what the tax system will look like in the future.