The Trump administration’s pledge to slash the corporate tax rate faces an immovable obstacle: how to pay for the lost revenue that would come from the cuts.

On the campaign trail, President Trump vowed to gut the corporate rate from 35 percent to 15 percent, a plan central to the administration’s larger pro-growth economic agenda.

Details of a road map for tax reform have to yet to emerge from the White House. But Gary Cohn, Trump’s director of the National Economic Council, recently told CNBC that the administration is committed to advancing tax reform under the budget reconciliation process, which will require any changes to the tax code to be deficit-neutral over the next 10 years.

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“We are actively working on a variety of different tax alternatives,” Cohn said, adding that the Trump administration will present a tax plan after health care reform is complete.

“We’re exploring all opportunities in taxes,” repeated Cohn.

Proposal to tax earnings in retirement plans

That pledge has some retirement policy experts predicting that the preferential tax treatment of employer-sponsored retirement plans will be considered as a way to offset revenue losses to cutting the corporate tax rate.

Last year, a proposal to lower the corporate tax rate to 15 percent was advanced by the non-partisan Tax Policy Center, a joint program led by the Urban Institute and the Brookings Institution.

The extensive plan includes a new 15 percent tax on the annual gains in 401(k) plans and other tax-preferred retirement accounts, which the proposal’s two authors—including one from the conservative leaning American Enterprise Institute—say would raise $48 billion in new revenues in 2018 and $60 billion in 2025.

Under existing law, capital gains and dividend earning on investments in retirement plans are not taxed until the savings are withdrawn, when they are taxed as regular income in most plans.

Letting the gains in retirement accounts go untaxed as the accounts grow contributes to the so-called forgone tax revenue in retirement plans.

Along with the tax breaks on plan participants’ income deferrals, excluding annual earnings from taxes accounted for nearly $159 billion in lost tax revenue in 2015, and more than $1 trillion in estimated foregone revenue between 2015 and 2019, according to the non-partisan Joint Committee on Taxation.

According to the Tax Policy Center’s proposal, the idea of taxing gains in retirement plans to support a 15 percent corporate tax rate is not designed “to expand or scale back tax benefits” for investors in retirement plans.

The objective is to keep the overall tax burden on retirement savers level with current law, according to the proposal.

In theory, it would limit the new net tax benefits savers would receive from a substantially lower corporate tax rate. At 15 percent, the U.S. would transform from the country with the highest corporate tax rate among the 35-member Organization for Economic Cooperation and Development, to a country with one of the lowest rates. “The value of corporate shares would likely rise in the short run,” the proposal says.

By other calculations, cutting the corporate tax rate would result in substantial gains for investors in qualified retirement plans.

Vinay Pande, a managing director at UBS, told CNBC that for every 5-percentage point reduction in the corporate tax rate, investors would realize a 4-percentage point increase in earnings per share growth. “These are very big, gigantic numbers,” said Pande.

Retirement advocates bracing for policy changes

Capping payroll and income tax breaks on some portion of deferrals to qualified retirement plans is not a new idea.

As a presidential candidate, Hilary Clinton proposed taxing 401(k) contributions for earners in higher tax brackets. Capping the total value of tax-deferred saving in IRAs was also floated by the Obama administration, as was a failed initiative to tax 529 college savings plans.

At the end of last year, the Congressional Budget Office published a comprehensive paper exploring ways to increase federal government revenue.

Reducing the annual contribution limits on 401(k)s to $16,000 and $5,000 for IRAs (catch-up contributions would be eliminated), and total contributions in defined contribution plans, including employer deferrals, to $47,000 from $53,000, would raise $97 billion in tax revenue from 2017 to 2026, according to the CBO.

All of the talk of revenue-neutral tax reform has some retirement advocates concerned.

At a policy forum hosted by the Employee Benefits Research Institute last December, Randy Hardock, a partner at Davis & Harman and a former staffer on the Senate Finance Committee, said the tax code’s preferences for retirement savings programs will be vulnerable under the administration’s effort to reform the tax code.

“It is incomprehensible to me that Congress will not go after retirement plans in some way in tax reform,” said Hardock at the forum.

Brian Graff, CEO of the American Retirement Association, reiterated the fear that the preferential treatment of retirement savings plans will be used to pay for overall tax reform.

Republicans “want to do tax reform that's revenue neutral, they want to lower rates, they want to lower taxes on investments, and they want to lower corporate rates,” Graff said at EBRI’s event. “To be revenue neutral, that money has to come from someplace else, and historically, retirement tax preferences have been a target to pay for other priorities.”