It's perfectly legal for billionaires to pay so little in taxes. Democrats say they could finally change that after the bombshell ProPublica report.

• A ProPublica report based on secret IRS files showed billionaires pay relatively little tax.
• Inequality experts have been warning for years that the wealthy pay relatively low taxes.
• The details added impetus to a push by Democrats to ramp up taxes on the country's highest earners.

On Tuesday morning, ProPublica published a bombshell report showing how little America's wealthiest pay in taxes, based on leaked documents from the Internal Revenue Service (IRS).

The report shows in detail how billionaires like Jeff Bezos and Warren Buffett have seen billions added to their net worth with little impact on their tax bill. It's totally legal, and for many, not all that surprising.

"It's not surprising at all, I think," Chuck Collins, who works at the left-leaning Institute for Policy Studies, an organization dedicated to highlighting wealth inequality, told Insider.

Collins recently wrote a book on the ways the ultrawealthy hide their money and avoid taxation. In it, he uses the term "wealth defense industry" for the cottage industry that's grown around helping the rich hold onto their money.

"It's going to be very hard for ordinary people to decipher these tax transactions because they're purposefully complex," Collins said. "The wealth defense industry, their bread and butter is complexity, and opaqueness."

Chuck Marr, the director of federal tax policy at the liberal-leaning Center on Budget and Progressive Priorities, said "we've been making this case for a long time." He pointed to a paper from 2019 that outlines many findings similar to those in Tuesday's report.

Still, it's one thing to know something is likely happening, and another to see the details laid bare, and the figures involved. For example, ProPublica found that Warren Buffett paid 0.1% in "true tax rate," which compares how much he paid each year in taxes to how much his wealth grew.

ProPublica's report could draw widespread attention - and scrutiny - to certain intricacies of the tax code just as President Joe Biden moves to reform taxes to pay for his infrastructure proposals.

Already, Democratic lawmakers are seizing on the public report as a way to kickstart tax reform.

The report "should make it very hard for the Congress to not address it," Marr said. "I think it really underscores, again, that very wealthy people do not pay tax on much of their income. And so this tax bill is a clear opening to address that."

America's wealthiest make most of their money from assets, not income

As the 2019 CBPP paper lays out, a good amount of the income that the wealthiest bring in isn't technically income - or at least it's not taxed that way.

If you work a job where you receive wages in a paycheck, you're probably familiar with the income tax, which taxes the money you get for going to work. Those wages would be income, and you'd be taxed under the income tax.

But, as both the CBPP and ProPublica note, the wealthiest Americans get most of their wealth from assets like stocks, and therefore pay taxes on capital gains.

As Marr and coauthors Samantha Jacoby and Kathleen Bryant write, capital-gains taxes are "effectively voluntary to a substantial extent: High-wealth filers may accumulate capital gains every year as their investments appreciate, but they don't owe tax on those gains until - or unless - they 'realize' the gain, usually by selling the appreciated asset."

So if you hold onto your stock assets, you're not seeing that capital gains rate. Goldman Sachs estimated last month that the wealthiest Americans possessed between $1 trillion to $1.5 trillion in unrealized capital gains at that time. Some argue that those unrealized gains should be taxed, since the wealthiest could be sitting on valuable stocks, making money, and not paying taxes. Meanwhile, researchers at the right-leaning Tax Foundation argue that a progressive consumption tax would be a better way to tax the rich.

ProPublica reported that the ultrawealthy can also borrow hefty sums of money to pay off their bills as they sit on stocks and take in little income. "They'll borrow money and they'll use the stock as collateral," Marr said. That means the wealthy are essentially using these loans as a form of income, but aren't taxed as such.

As Marr, Jacoby, and Bryant write, "this is often a much cheaper strategy than selling stock and paying capital gains taxes, particularly when interest rates are low."

The report could add flame to the fire for tax reform

Even before the ProPublica report, tax debate had been brewing. In particular, a provision called the "step-up basis" had been facing scrutiny.

Let's say you've held onto stock for your whole life, and it's only grown in value. If you die and leave it to someone else, the stock takes on the value at which the recipient gets it, meaning neither the original owner nor the inheritor are taxed on those gains.

For very wealthy people, Marr said, that "wipes out a lifetime of tax liability."

Biden wants to do away with the step-up basis and he wants to tax capital gains for those making over $1 million at a rate equivalent to income.

"Broadly speaking, we know that there is more to be done to ensure that corporations, individuals who are at the highest income are paying more of their fair share," White House Press Secretary Jen Psaki told The Washington Post in response to the ProPublica report. "Hence, it's in the president's proposals. His budget and part of how he's proposing to pay for his ideas will go ahead."

"The principle here is to equalize the treatment of ordinary income and capital gains, and that is a principle that's neither new or particularly novel," Brian Deese, the director of the National Economic Council, said in an April briefing. "In fact, the last president to enact a reform to equalize the treatment of ordinary income and capital gains was President Reagan, who did so while raising capital-gains taxes as part of the 1986 tax reform."

The White House did not respond to Insider's request for comment.

There's been GOP resistance to further alterations to the tax code following their 2017 tax cut, especially any increase in rates. But the new reporting already ramped up the tax debate within Congress on Tuesday.

Sen. Bernie Sanders, who chairs the Senate Budget Committee, told reporters on Capitol Hill, "To the surprise of nobody I know, the rich and powerful aren't paying their fair share, what else is new?" He urged lawmakers to approve Biden's tax proposals.

"I do want people to understand the bottom line," Sen. Ron Wyden, chair of the Senate Finance Committee, told reporters. "What ProPublica is revealing is, again, some of the country's wealthiest taxpayers [that] profited handsomely during the pandemic are not paying their fair share."

He said he's in the process of crafting a proposal to change that. Asked by Insider about the timeline of its introduction, Wyden responded: "I'll have it ready to go shortly."

"Often solutions to this are portrayed as radical, but what's radical is the current situation," Marr said. "What's radical is that wealthy people, a lot of their income never gets taxed. That's radical."