While the Treasury Department estimates a book tax could raise $148.3 billion over a decade, the idea has some downsides. It can blunt the effectiveness of some tax preferences that Democrats are also pursuing, including incentives for domestic manufacturing and credits for renewable energy. If companies have to pay a minimum rate, they can be boxed out of claiming some or all of those tax breaks, reducing the intended policy outcomes.

Foreign Profits
House and Senate Democrats are largely unified in wanting to strengthen the tax rules on U.S. companies operating abroad, both by collecting more tax on profits they earn offshore and increasing penalties on firms that shift revenue and operations overseas. The two chambers have competing proposals, though they address the same issues with minor technical differences.

However, Democrats would need to take a much more aggressive approach if they want to raise revenue to cover some of the gap from a company, according to Steve Wamhoff, the director of federal policy at the Institute on Taxation and Economic Policy, a left-leaning think tank.

For example, Biden has floated a 21% minimum rate on overseas earnings, while the House version of the bill scaled that back to 16.56%. In total, the House bill raises about $388.7 billion from increasing taxes on multinational corporations, compared with the $533.5 billion in Biden’s initial plan—a $164.8 billion difference.

However, it could be difficult to get moderate Democrats to agree to such substantial changes.

Stock Buyback Tax
Democrats are also thinking about implementing a stock buyback tax, a new levy that would equalize the tax treatment for companies when the buy back their own shares and when they issue dividends to stockholders. A plan, authored by Senate Finance Committee Chairman Ron Wyden and Senate Banking Chair Sherrod Brown, would apply a 2% excise tax on those purchases.

Brown’s office said the tax could raise more than $100 billion over a decade, but the idea, which is one of the more novel and untested among those under consideration, is causing some uneasiness.

“Any time you get into something that’s not proven in the tax code it gets a bit dangerous,” Senator Jon Tester, a Montana Democrat, said.

Executive Compensation
Another area Democrats could target is the high pay for chief executive officers. House Democrats in their bill put limits on how much firms can write-off for the compensation of their top executives.

The House provision raises about $16.9 billion over a decade. Even if Democrats were to expand that proposal, it could only generate a small fraction of the revenue needed to cover the corporate-tax gap.