• The U.S. Chamber of Commerce and the Business Roundtable are gearing up to fight new tax proposals in Congress as Democrats work to finish President Joe Biden's social safety net and climate spending bill.
• The proposals would target billionaires through the tax code and enact a 15% minimum corporate tax on declared income for large corporations.
• Centrist Democrat Joe Manchin has voiced some concerns about the so-called billionaire tax.

Two of the biggest pro-business groups in the country — the U.S. Chamber of Commerce and the Business Roundtable — are gearing up to fight new tax proposals in Congress as Democrats work to finish President Joe Biden's social safety net and climate spending bill.

The proposals would target billionaires through the tax code and enact a 15% minimum corporate tax on declared income for large corporations.

Centrist Democrat Joe Manchin has voiced some concerns about the so-called billionaire tax. The senator from West Virginia, who represents a pivotal vote in the Senate, has worked to shrink Biden's and Democrats' proposals for the bill. He favors the minimum corporate tax, however.

The Chamber, the biggest business advocacy group in the country, told CNBC that it is concerned that lawmakers are rushing these proposals through Congress.

"We are concerned about this attempt at the 11th hour to rush through brand new, complex, and untested methods of taxation," Neil Bradley, the group's executive vice president and chief policy officer, said in a statement. The group, he said, thinks lawmakers haven't thoroughly vetted the proposals for unintended consequences.

"No lawmaker can say they have seriously contemplated the impact of these proposals," Bradley said.

The Business Roundtable, which includes many of the country's top corporate CEOs, opposes the 15% minimum corporate tax on large corporations, a spokesperson said.

"Business Roundtable will continue to lobby against harmful tax increases on America's job creators, including the 15% 'book minimum tax' currently being discussed," the spokesperson said. The group says the proposal would impact tax incentives that promote investment in plants and equipment, while undermining the quality of financial reporting.

Members of the lobbying group include Walmart CEO Doug McMillon, who is the group's incoming chair, and General Motors CEO Mary Barra. Tim Cook, the CEO of Apple, and Jamie Dimon, the CEO of JPMorgan Chase, are also members.

Democrats have proposed the new tax provisions as a way to raise revenue for their spending bill.

On Tuesday, Senate Democrats introduced the idea of initiating a 15% minimum corporate tax on declared income of massive businesses, such as Amazon, Facebook and Tesla. The other idea is to raise money through a billionaire tax that would target unrealized gains. It would hit those with more than $1 billion in assets or incomes of more than $100 million each year.

It's unclear whether the billionaire tax will pass Congress. Rep. Richard Neal, D-Mass., chairman of the powerful House Ways and Means Committee, told reporters Wednesday that the proposal is "apparently dead." Yet, Sen. Ron Wyden, D-Ore., chair of the influential Senate Finance Committee, insisted the proposed tax isn't dead yet. Wyden introduced the proposal this week.

The Business Roundtable and the Chamber of Commerce have already spent millions lobbying Congress on a wide variety of issues this year, including infrastructure reform, taxes and trade, according to lobbying disclosure reports reviewed by CNBC.

The two groups support Biden's $1 trillion infrastructure bill but are against the reconciliation package. The Chamber reiterated its opposition to the social spending bill earlier this month. The Business Roundtable said in September that it was against including tax increases in the measure.

In the third quarter alone, the Chamber spent over $16 million and the Business Roundtable spent over $11 million lobbying Congress, the White House and other parts of Biden's administration.

The two lobbying giants are not alone in the business community in objecting to the new tax concepts.

Tesla CEO Elon Musk took aim at the billionaire tax in a tweet late Monday. Musk, according to Forbes, is the richest person in the world, with a net worth of about $275 billion.

In an interview with CNBC, billionaire investor Ron Baron criticized Congress for its proposals and defended Musk over the new tax ideas.

"It feels like Congress thinks they are Robin Hood, from the old days. It feels like the Elon Musk tax bill," Baron said on Wednesday. "I don't really understand why there is vindictiveness against someone who works the way he [Musk] has."

Baron has a net worth of over $4 billion, according to Forbes.