Nearly a year after its enactment, the U.S. government still has not yet fully implemented the new corporate alternative minimum tax, as the Biden administration races to finalize a complex and critical element of Democrats’ broader economic agenda. Its fate rests in the hands of the Treasury Department, whose forthcoming rules will determine if Biden can achieve his promises to lower the federal deficit and force businesses to pay their fair share.

“It’s my sense we’re going to raise less revenue from the tax … than we initially hoped,” said Jeffrey Hoopes, a tax professor at the University of North Carolina, who previously warned Congress that its vision may not translate to good policy.

Democratic lawmakers envisioned the new corporate minimum tax as a way to pay for their signature spending package, the Inflation Reduction Act, which included costly new programs to combat climate change and lower health-care costs. Party leaders explicitly targeted a small set of companies that regularly report massive, multibillion-dollar profits to shareholders yet remit little to the government each year.

In a fiscal review of the policy, the nonpartisan Joint Committee on Taxation estimated last year that about 150 corporate taxpayers could be forced to pay the new tax, later finding it could generate more than $222 billion in federal revenue over the next decade. But exactly which firms pay — and how much money the government takes in — hinges on a series of unresolved legal issues that stem from a law written in haste.

In the meantime, major lobbying organizations that represent companies like AT&T, Amazon, Duke Energy, Ford and FedEx have seized on the uncertainty to press for changes that may reduce their tax bills. One group representing gas utilities even warned they could raise prices on customers depending on how the administration implemented the tax rules. (Amazon founder Jeff Bezos owns The Washington Post, and interim Post CEO Patty Stonesifer sits on the company’s board.)

The developments have raised fears among some watchdogs that savvy corporations could end up paying far less than Biden envisioned, skirting a policy that explicitly aims to prevent profitable firms from dodging taxes.

“These lobbies exist not to create clarity in the tax law, but to reduce the tax liability of the companies they represent,” said Matt Gardner, a senior fellow at the Institute on Taxation and Economic Policy, a left-leaning group.

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Two years earlier, the organization found that 55 companies — including Duke, Nike, the steelmaker Nucor and Salesforce — each paid nothing in taxes on their 2020 profits, a figure that Biden repeatedly cited as he made the case for a sweeping overhaul. While he initially sought to raise the overall corporate rate to 28 percent from 21 percent, the president ultimately settled on a series of lesser policies targeting corporate profits.

One new tax applies to companies that repurchase their stock. Since its adoption, companies including Apple, Google and Wells Fargo have announced a series of sizable buybacks, prompting Biden recently to call on Congress to quadruple the tax to 4 percent.

The other is the new 15 percent corporate minimum, initially championed by lawmakers including Sen. Elizabeth Warren (D-Mass.) and modeled in some ways after a similar effort in the 1980s. Touting the tax over the past year, Biden has charged it is “wrong” that businesses can use generous deductions under the existing tax code — sometimes reducing their bills to zero — while teachers, firefighters and other workers “pay more than that.”

Generally, companies that earn $1 billion or more per year, averaged over a three-year period, are supposed to owe the tax. Large firms are expected to compute what they would normally pay after accounting for deductions that can reduce their tax bill, then conduct an alternative calculation based on their financial statements, known as book income. They essentially are expected to pay the larger of the two.

But Congress left critical details up to the Treasury Department, which issued its initial guidance last year. The document — little noticed outside corporate boardrooms and large accounting firms — signaled the agency is still setting the terms for how companies should determine their income, and more broadly, which policies in the byzantine corporate tax code should apply to the new system.

“We value stakeholder input as we’ve worked on this project but we keep as our north star congressional intent, which is the idea of making sure we make the tax code more fair,” said Deputy Treasury Secretary Wally Adeyemo.

With so much to resolve, though, the IRS announced in June that it does not expect companies to begin estimating and paying the new tax yet on a quarterly basis — and, as a result, would not assess penalties on those who failed to do so. Tax experts described the pause as a common practice, albeit one that has illustrated the complexity of the task the government now faces.

“There are virtues to the [corporate alternative minimum tax], but I don’t think anyone with a straight face thinks that simplicity is one of them,” said Chye-Ching Huang, the executive director of the Tax Law Center at the NYU School of Law. “It’s a lot of decisions Congress has left to Treasury and the IRS to figure out in some of the hardest areas.”

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Already, a widening array of firms — including Netflix, Salesforce and Warner Bros. Discovery — have told investors they are evaluating if they might face new federal tax liabilities under the law. The insurer AIG warned in February it “may be required to pay” the tax.

In an early attempt to pinpoint who might be liable, Hoopes and researchers at UNC studied companies’ financial statements up to 2021 — before lawmakers adopted the Inflation Reduction Act — and found 78 firms that year would have faced $31.8 billion in liabilities. That included Berkshire Hathaway, the conglomerate owned by Warren Buffett, which may have owed $8.3 billion in taxes under the law. Amazon would have faced a roughly $2.7 billion bill, Ford Motor Co. would have owed $1.8 billion and AT&T might have had to pay $1.5 billion, according to the analysis, released last September.

Hoopes cautioned in a recent interview that the study was only a projection, as corporate tax filings are not public. It also hinged on an assumption that Treasury might recognize for tax purposes the gains and losses in the value of some corporate assets and investments, even if they are not actually sold.

Like much of the law, Congress left that matter somewhat ambiguous, though Hoopes said the government since then has signaled it is disinclined to do so, which might limit the reach — and possibly the revenue-raising potential — of the new tax.

“The vast majority of [earnings reports] I see say, ‘We don’t expect to be subject to this,’” Hoopes said.

At least one firm, Berkshire Hathaway, later reported it “currently [does] not expect that compliance with the provisions of the 2022 act will have a material impact” on its finances, according to its most recent earnings report. The company did not respond to a request for comment.

For the Treasury Department, the fiscal stakes are high, since Democrats fashioned the tax as a way to ensure the Inflation Reduction Act reduces the deficit. Since its adoption, though, federal budget watchdogs have projected its price tag could reach as high as $660 billion, more than 60 percent higher than early estimates, according to the JCT.

The spike is due in large part to higher-than-expected demand for clean energy tax credits, including those that encourage consumers to purchase electric vehicles. Congress also included a series of carve-outs that allow companies to use some research and investment expenses to offset their taxes, mirroring the existing code in a way that could lower corporate tax bills.

And lawmakers further agreed to shield entire industries — including real estate conglomerates and wealthy private-equity firms — from the new tax. Sen. Kyrsten Sinema (Ariz.), now an independent who caucuses with Democrats, pushed to protect private equity as part of a last-minute deal to win her must-have vote.

“There were all these other adjustments that came along … that made this tax look more and more like the regular corporate tax,” said Will McBride, the vice president of federal tax and economic policy at the Tax Foundation, a right-leaning organization. That, he said, threatened to depress revenue — and raised the prospect that taxpayers could ultimately face “an effective tax rate that’s lower than the statutory tax rate.”

Some corporations in recent months have shifted their lobbying to the Treasury Department, urging the agency to take a lenient hand — or risk a potential threat to economic growth.

“There are reasons and incentives that Congress gave businesses to do certain good things in the economy,” said Lara Muldoon, the senior director for government affairs at the Information Technology Industry Council, a trade group that represents Amazon, Facebook, Google and other technology giants.

In comments filed with the agency this spring, a wide array of corporate lobbyists — including ITI, the Business Roundtable and the Alliance for Competitive Taxation (ACT), which represents companies like Google, Home Depot and Walmart — pushed for permissive rules that mirror many of the features of the current tax system. They essentially sought permission to use a wider array of business expenses as offsets to their income, and called on the government not to expose some of their investments to new taxes.

“We want to make sure that for items that have effect in pre-IRA years … you’re not getting penalized for them because you made decisions before anyone ever thought a book tax would be implemented,” said Chris Netram, the head of policy at the National Association of Manufacturers, whose leadership includes executives from Caterpillar, Dow, ExxonMobile, Johnson & Johnson and other major companies.

Business Roundtable declined to comment, and ACT did not respond to a request.

The American Gas Association, which represents utilities, sought to ensure the minimum tax allowed companies to take a series of deductions under the existing tax code that benefit the construction and repair of their facilities. Those deductions “provide a substantial benefit to the customers of their companies,” the group wrote, warning that a failure to incorporate them “will void this customer benefit and result in substantial increases to customer rates.” The group did not respond to a request for comment.

As the Biden administration crafts its rules, some opponents similarly have reengaged the issue on Capitol Hill, according to two congressional Democratic aides who spoke on the condition of anonymity to describe private discussions. Oil companies, for example, have sought special treatment for new drilling costs under the minimum tax, one of the people said, even though the Inflation Reduction Act aimed to reduce the country’s dependence on fossil fuels.

Representatives for major private-equity firms, including Blackstone and the Carlyle Group, similarly have blitzed lawmakers attempting to ensure they are not taxed based on the gains and losses of assets they have not sold, according to Democratic aides.

Blackstone did not respond to a request for comment, and a spokeswoman for Carlyle Group declined to comment. In its most recent earnings report, Blackstone signaled it “expects to have” a liability in 2023, while Carlyle indicated in its annual report this March that the tax provisions broadly in the Inflation Reduction Act “have not had and are currently not expected to have a material impact” on its finances.

Even in the face of intense corporate lobbying, experts say they still expect Treasury to offer additional guidance about the tax — how it applies, and to whom it applies — before the end of the year. In the meantime, top administration officials have promised that their labors still can erase decades of corporate tax dodging.

“By ensuring large, profitable companies and high-income individuals pay their fair share through these provisions and others, we are making our tax system and economy work better for everyday Americans,” said Lily Batchelder, the assistant treasury secretary for tax policy, in a speech in San Diego earlier this year.