At the Supreme Court Tuesday, the justices approached a major tax case with all the concern that might have greeted an unexpected ticking package on the front porch. The justices' apprehension is likely justified because their eventual decision in the case could severely limit congressional options in enacting tax policy, and it could cost the federal government trillions of dollars in corporate taxes.

The case before the court is widely seen as a preventive strike against Sen. Elizabeth Warren's wealth tax--not that her proposal has any real chance of being enacted.

But the tax under the judicial microscope Tuesday was enacted in 2017 in part to fund President Trump's massive corporate tax cut. Called the Mandatory Repatriation Tax, or MRT, it imposed a one-time tax on off-shore investment income.

For Charles and Kathleen Moore, that meant they owed a one-time tax of $15,000 on a investment in India--an investment that grew in value from $40,000 to more than $500,000. The Moores paid the tax and then challenged it in court, contending that the tax violates the Sixteenth Amendment to the Constitution, which authorizes Congress to impose taxes on income.

What the federal government can tax

In the Supreme Court chamber Tuesday, the Moores' lawyer, Andrew Grossman, told the court that the federal government can only tax income that is actually paid to the taxpayer—what he called "realized income," as opposed to the Moores' "unrealized income."

Chief Justice John Roberts noted that the corporation in which the Moores invested certainly has realized income. And Justice Sonia Sotomayor asked about the many other ways that investments are taxed, even though there is no pay-out to individuals. These include everything from real estate partnerships to law firms.

"Why do we permit taxing of individual partners" even though "a partner doesn't have personal ownership, doesn't get the value of the partnership, yet we've permitted that tax?"

Grossman replied that "a partnership is a fundamentally different form of organization than a corporation."

Justice Elena Kagan pointed to the country's long history of taxing American shareholders' on their gains from foreign corporations.

"There is quite the history in this country of Congress taxing American shareholders on their gains from foreign corporations and you can see why, right?" Kagan asked. "Congress, the U.S. Government can't tax those foreign corporations directly, and they wanted to make sure that Americans didn't... stash their money in the foreign corporations, watch their money grow, and never pay taxes on them."

And Justice Brett Kavanaugh chimed in with this observation: "We've long held that Congress may attribute the income of the company to the shareholders or the partnership to the partners."

The government's position

Defending the tax, Solicitor General Elizabeth Prelogar faced a grilling from both Justices Samuel Alito and Neil Gorsuch.

"I'm just asking what the limits of your argument are?" said Gorsuch, adding, "It seems to me there are none."

Prelogar replied that under the Constitution, "Congress has broad taxing power." Indeed, she pointed to the Supreme Court's own decisions saying that "Congress has plenary power. It can tax people just for existing."

By the end of the argument Prelogar seemed to have assuaged some of Gorsuch's fears.

"The reason why I would strongly caution the court away from adopting a realization requirement is not only that we think that it is inaccurate, profoundly ahistorical, inconsistent with the text of the Sixteenth Amendment," she said. "It would also wreak havoc on the proper operation of the tax code."

Former Republican House Speaker Paul Ryan, who shepherded the 2017 tax bill through the House made a similar point in September, warning that if the MRT is invalidated, it could unravel a third of the tax code.