President Joe Biden's administration could unveil a broad tax reform package as soon as April that would deal a substantial hit to earnings for S&P 500 companies, particularly those in tech-focused industries.

But investors have largely ignored the looming hike in U.S. corporate taxes, perhaps because the White House's plans are still in flux and questions remain if the slim majority held by Democrats in Congress is enough to push through any meaningful change.

As a presidential candidate, Biden proposed raising the corporate tax rate from 21% to 28%. The rate was lowered from 35% to 21% in a 2017 tax law pushed by the Trump administration and congressional Republicans.

Biden's team has also proposed a new 15% corporate minimum tax on companies with more than $100 million in annual income and doubling taxes on foreign earning through the Global Intangible Low-Taxed Income, or GILTI, rate to 21%.

If these tax proposals were fully implemented, earnings per share of S&P 500 companies would see a decline of about 9% in 2022, a team of Goldman Sachs analysts wrote in a March 19 note.

"But the eventual impact will depend on the specifics," the team led by David Kostin, Goldman's chief U.S. equity strategist, wrote. "A dearth of details makes it hard to model and trade on the potential earnings impact of legislation that hasn't yet been outlined."

Assuming an increase in the statutory corporate tax rate from 21% to 28%, Goldman economists forecast a 3% drag to earnings by 2022.

Other possible hikes under consideration, including a doubling of the GILTI rate and imposition of a minimum corporate tax rate, pose further downside risks.

That risk would vary by sector as well. For example, Kostin's team sees a minimum tax rate on foreign earnings posing the largest threat to low tax "growth" sectors, such as information technology and healthcare. Domestic-focused sectors, such as industrials and materials, would face much less of a hit to earnings, they said.

Information technology constituents of the S&P 500 would see a 4% impact to EPS if the corporate statutory tax rate was increased from 21% to 28% and a 6% hit to EPS if the GILTI tax rate was also increased from 11% to 21%, according to Kostin's forecast. The communication services sector would also face a 10% EPS hit, including a 6% decline from the corporate tax increase and a 4% decline from the GILTI tax hike.

Meanwhile, companies in the energy, financials and utilities sectors would face EPS hits next year of between 6% and 8% from the corporate tax increase, but no impact from a GILTI tax increase.

The tax hikes are intended to partially fund a more than $2 trillion infrastructure spending plan the Biden administration is developing. That price tag could roughly double if other healthcare, education and childcare initiatives are attached.

According to Kostin, while investors are "anxiously focused" on rising bond yields and inflation, concerns of increased tax rates have not been priced into stocks of companies that could be hit hardest by a change.

"After trading closely with prediction market odds during the months ahead of the general election in November 2020, a pair of tax baskets containing the biggest winners and losers from the 2017 tax cuts has reversed and now appears to be pricing little risk of higher tax rates," Kostin wrote.

The hazy policy path forward is also clouded by just which tax proposals will become a priority.

"We expect an increase in the corporate tax rate to 25% and an increase in corporate international taxation, but we think the jury is out on most other tax items," James Lucier, a managing director at Capital Alpha Partners, wrote in a March 19 note.

In a March 8 analysis, Brian Gardner, chief Washington policy strategist with Stifel, said the probability of an increase in corporate income taxes from 21% to 28% was high, although moderate Democrats in Congress might push for a smaller increase, perhaps to only 24%.

Gardner sees a doubling of the GILTI rate from 10.5% to 21% as highly likely but a 15% corporate minimum tax on companies with over $100 million in annual income as less likely.

Any tax hike is unlikely to get Republican support and the issue is likely to divide centrist Democrats and progressives, frustrated that an increase to the minimum wage was dropped from Biden's latest stimulus package.

"The tension between progressives and centrists could become more intense as a result," Gardner wrote. "Given Democrats' razor-thin majorities in both the House and Senate, any dissension could be fatal to the bill."