A massive trove of tax information obtained by ProPublica, covering thousands of America’s wealthiest individuals, reveals what’s inside the billionaires’ bag of tricks for minimizing their personal tax bills — sometimes to nothing.

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The Democratic plan to crack down on individual retirement accounts worth hundreds of millions, even billions, of dollars and to tighten the rules governing IRA investments is facing intense opposition from several industry groups seeking to kill or soften the proposed reforms.

Several retirement industry firms, including one backed by tech investor Peter Thiel, who amassed a multibillion-dollar IRA, have mounted a lobbying push against the plan, disclosure filings show. They have hired an array of former Capitol Hill staffers, a former congressman and at least one former U.S. senator to fight efforts to rein in and regulate the accounts.

ProPublica reported in June that, as of 2019, Thiel had $5 billion in one of the tax-advantaged accounts, which were originally intended to incentivize retirement savings by the middle and working classes. Other superrich Americans have also protected large fortunes from taxation using the retirement vehicles. Income generated inside an IRA is not taxed and, in the case of a Roth IRA such as Thiel’s, withdrawals are also tax-free once the owner reaches the age of 59 and a half.

In response, Democrats on the House Ways and Means Committee included a proposal to curb mega-IRA accounts as part of the package of social spending and tax changes being debated on Capitol Hill. The House proposal would effectively cap the total amount someone could hold in a Roth IRA at $20 million and compel the holders of the giant accounts to withdraw anything over that limit. “Incentives in our tax code that help Americans save for retirement were never intended to enable a tax shelter for the ultra-wealthy,” Ways and Means Chair Rep. Richard Neal, D-Mass., said earlier this year. “We must shut down these practices.”

The proposal would also bar IRAs from making certain nonpublic investments, an area that congressional investigators have flagged as ripe for abuse. Purchasing shares of startups through an IRA has become popular among the founders of Silicon Valley companies. Buying difficult-to-value shares at extremely low prices can sidestep IRA contribution limits and potentially generate massive tax-free growth.

Among the companies lobbying on the proposal is San Francisco-based Forge Global, which runs a marketplace for shares of private companies and also has a division that administers IRA plans. Forge hired lobbyists from Allon Advocacy in September, including former staffers for Sen. Rob Portman, R-Ohio, and former Rep. Gary Ackerman, D-N.Y.

Thiel was one of the early funders of Forge, and it regularly touts its connection to him in press releases. Forge CEO Kelly Rodriques was previously the chief executive of an IRA company formerly called Pensco. Pensco was the custodian of Thiel’s giant IRA for many years, ProPublica previously reported.

A Thiel spokesperson didn’t respond to a request for comment. A Forge spokesperson said Thiel and his representatives had no role in the company’s decision to lobby on the IRA proposal. She said that while Thiel is an investor in the company, he is not an adviser or on its board. “Our engagement on this issue has been focused narrowly on two provisions … that we believe will have unintended negative consequences on middle-class Americans who are saving for retirement,” she said.

The firm supports a “reasonable cap” on IRA account sizes, she said, but takes issue with provisions that would restrict what investments IRAs can make.

It’s not clear how many IRAs would be affected by the proposal’s restrictions on certain IRA investments. The Government Accountability Office, the investigative arm of Congress, reported that, in 2016, only about 2% of IRAs held so-called unconventional assets, such as stakes in companies that are not publicly traded. Only a subset of those accounts would be affected by the reform proposal. The GAO also noted that very large retirement accounts often held these assets.

Several other retirement industry companies are also mobilizing to fight the proposal by backing a new group called the Individual Retirement Rights Association. It was formed in late August in Delaware, state records show. It hired the firm Crossroads Strategies to lobby on the IRA proposal. Disclosure filings show a team of eight lobbyists working for the group, most notably former Sen. John Breaux, a Democrat who represented Louisiana.

After leaving the Senate in 2005, Breaux launched his own lobbying outfit with retired Republican Sen. Trent Lott of Mississippi. When in Congress, Breaux once famously commented that while his vote could not be bought, it could be rented. Crossroads didn’t respond to a request for comment.

Another former Democratic senator, Max Baucus of Montana, wrote an op-ed in USA Today earlier this month sharply criticizing the IRA proposal. Baucus, who was also ambassador to China during the Obama administration, now has his own consulting shop, the Baucus Group. He is not a registered lobbyist. He didn’t respond to requests for comment.

A preexisting trade group called the Retirement Industry Trust Association is also advocating to weaken the proposal but says it “supports a reasonable and sensible cap” on retirement account sizes. The group said it opposes a change in the law that “limits investment choice.”

A harder line is being pursued by the Taxpayers Protection Alliance, an anti-tax group that is affiliated with the Koch political network. It launched a new “Don’t Touch My IRA” campaign, saying that “President Biden is proposing draconian changes to retirement savings in IRAs and Roth IRAs as part of a desperate effort to pay for $3.5 trillion in irresponsible spending.”

The group also hired lobbyists to fight the measure, including former. Rep. Ben Quayle, an Arizona Republican and son of the former vice president. A spokesperson didn’t respond to a request for comment.

Passing any package of social spending and tax changes will require the votes of every Senate Democrat, and the Biden administration is currently engaged in intense negotiations to craft a bill that can pass. That means that, like everything in the package, the mega IRA proposal is vulnerable to being killed or changed if just a single Democratic senator demands it.

One congressional staffer with knowledge of the recent negotiations said that Sen. Kyrsten Sinema, D-Ariz., has a list of “approved” policy changes, and the IRA proposal is not on it. Sinema has not publicly commented on the proposal and a spokesperson didn’t respond to a request for comment.