Donald Trump's campaign is working on a response to the latest Hillary Clinton attack on his economic plan, namely the charge that he would open up a "Trump loophole" in the tax code that would cut taxes for him and other ultra-wealthy people.

It's a real problem with the Republican presidential candidate's proposed tax reform, one that his advisers are seeking to fix. The loophole is one that has helped stymie Republican efforts for lowering tax rates, although House Republicans now believe they have found a fix.

The problem, as Clinton laid it out in a speech on the economy in Detroit on Thursday, is that the "Trump Loophole ... would allow him to pay less than half the current tax rate on income from many of his companies."

In the update to his tax plan unveiled this week, Trump announced he would cut the top tax rate for the individual income tax to 33 percent and the tax rate for all business income to just 15 percent.

By setting the tax rate on business income so far below the rate on labor income, Trump would create a few big distortions and loopholes, most tax experts believe.

One such wrinkle is that doing so would open up a big advantage for the businesses that file through the individual side of the tax code.

Such "pass-through" businesses, such as partnerships, S-corporations and sole proprietorships, are actually more significant than C-corporations are today: They account for more than 60 percent of business income and more than half of employment. Trump's businesses are mostly structured as pass-throughs, according to his presidential financial disclosures.

And while those pass-through companies are mostly small businesses and people doing contracting work on the side, many are not. The Center for American Progress, a left-leaning think tank with ties to the Clinton campaign, reported in a new study this week that 100,000 pass-throughs are big businesses and that 70 percent of partnership and S-corporation income, most of which is through the financial industry, accrues to the top 1 percent of income earners.

Trump would tax those businesses the same as C-corporations, meaning at 15 percent. Today, they are taxed at a top rate of 39.6 percent, plus state and local income taxes. That is within the range of the top tax rates owners of C-corporations pay on business income, when both the 35 percent corporate tax rate and the dividend tax are collected.

In other words, Trump's plan to cut the tax rate for pass-through income to 15 percent would be a major tax break for Trump's own companies, as well as for high income earners.

But there's an additional problem associated with Trump's plan. Because the tax on pass-throughs would be so much lower than the individual income tax in his plan, it would create an incentive for many professionals to claim that they are contractors rather than employees to pay the lower rate.

Such concerns are not ill-founded. When Kansas exempted pass-throughs from corporate taxation in 2012, the number of business entities structured as pass-throughs soared, costing the state millions in revenue and creating a budget hole that the Republican government has struggled to fill.

Nor is the Trump team unaware of the problem. Adviser Stephen Moore, a prominent supply-side economist at the conservative Heritage Foundation, told Bloomberg that the campaign is "absolutely dedicated to making sure the 15 percent is for legitimate businesses." More details, he added, will be coming in a matter of weeks.

House Republicans already have wrestled with a similar issue in their own tax reform plan, which has inspired Trump to revise his own plan to bridge the differences between the two.

The GOP plan, ordered by House Speaker Paul Ryan and being fleshed out by House Ways and Means Chairman Kevin Brady of Texas, also aims for a top individual tax rate of 33 percent, but would lower the corporate income tax rate to 20 percent, meaning that there would be a smaller gap between the two. Accordingly, there would be less pressure on individuals to say that their earnings are really business income.

But the GOP plan also includes safeguards to prevent gaming of the different rates. It would impose rules saying that owner-operators pay themselves "reasonable compensation," taxed at normal individual rates, rather than claim that all their company's earnings were business income.

For Trump, some similar sort of policy will be necessary to navigate the tricky politics of tax reform. Not cutting pass-through income is not an option: Small business groups were among those that made it impossible for Republicans and the Obama White House to find common ground on business tax reform last year, because they feared that they would be left behind.