To help pay for his big economic and social agenda, President Joe Biden is looking to go where the big money is: billionaires.

Biden never endorsed an outright "wealth tax" when campaigning last year. But his more conventional proposed rate hikes on the income of large corporations and the wealthiest Americans have hit a roadblock.

That leaves a special tax on the assets, not the income, of billionaires being proposed by a Senate Democrat as a possible vehicle to help pay for child care, universal pre-kindergarten, child tax credits, family leave and environmental initiatives.

Biden has vowed...