New proposals would fund social and climate programs by tapping billionaires’ unrealized gains and by ensuring that the biggest companies cannot avoid income taxes altogether.

WASHINGTON — Senate Democrats rushed on Tuesday to nail down the details of a groundbreaking tax on billionaires’ wealth, part of an elaborate menu of tax increases to finance a significantly scaled-back bill that would strengthen the social safety net and address climate change.

Among them is a measure Senate Democrats presented on Tuesday that would impose a 15 percent minimum tax rate on corporations based on the profits they report to their shareholders, not what they show to the Internal Revenue Service.

The billionaires’ tax and the corporate minimum tax faced skepticism among House Democrats, who questioned their feasibility, and both were likely to encounter legal and constitutional challenges.

Biden’s plan, serving as a one-woman blockade against more conventional tax rate increases, Democrats appeared to have no choice but to turn to creative revenue measures.

Democratic leaders hoped to unveil a final bill on Wednesday that could pass the House and Senate, but several sticking points remained.

“I’d rather we put programs out there, and if people like them, then we should continue them as a government, and if for some reason they’re not popular, well, then that also helps make some determinations,” said Representative Mark Pocan, Democrat of Wisconsin and a leader of the progressive House Democrats.

That left the tax increases that Democrats were cobbling together on the fly as potentially the most far-reaching aspect of the plan.

Sinema — to raise the corporate income tax rate to at least 25 percent from 21 percent, still far lower than the 35 percent rate paid before President Donald J.

Under the plan, companies with at least $1 billion in profits — about 200 publicly traded corporations — would no longer be able to escape income taxation altogether.

Senate Finance Committee aides singled out Amazon, which over the last three years reported $45 billion in profits, including a record $20 billion last year, but paid an effective tax rate of 4.3 percent.

Sinema blessed it as “a common-sense step toward ensuring that highly profitable corporations — which sometimes can avoid the current corporate tax rate — pay a reasonable minimum corporate tax on their profits.”.

The 16th Amendment clarified that income taxes do not have to be apportioned, and Mr

Wyden was careful to say his billionaires’ tax was a tax on income, not wealth: “You can’t have wealth without income,” he said