Sen. Joe Manchin (D-W.Va.) signaled on Monday that he's open to a tax targeting billionaires as Democrats scramble to figure out how to pay for their sweeping social spending bill.

"I'm open to any type of thing that makes people pay, that's not paying now. So people don't report income like you and I do, earned income, there has to be a way for them to pay their fair share," Manchin told reporters when asked about supporting a billionaire tax.

Manchin added that while there were different approaches among Democrats, he supported "everyone basically paying their fair share of taxes."

Senate Finance Committee Chairman Ron Wyden (D-Ore.) is working on a so-called billionaire tax that would place an annual tax on unrealized capital gains.

The Wall Street Journal reported over the weekend that the proposal would affect people with $1 billion in assets or $100 million in income for three consecutive years.

“I think what's under consideration is a proposal that Senator Wyden and the Senate Finance Committee have been looking at that would impose a tax on unrealized capital gains, on liquid assets held by extremely wealthy individuals, billionaires. I wouldn't call that a wealth tax,” Treasury Secretary Janet Yellen told CNN on Sunday.

“But it would help get at capital gains, which are an extraordinarily large part of the incomes of the wealthiest individuals, and right now escape taxation, until they're realized, and often they're unrealized in the death benefit from a so-called step up of basis,” she added.

Democrats are racing to come up with a plan on how to fund their package, which is central to Biden’s legislative agenda and includes key investments in child and health care, among other initiatives.

The idea of a billionaire tax appears to be gaining momentum as Democrats tried to figure out a way around Sen. Kyrsten Sinema's (D-Ariz.) opposition to raising tax rates.

Democrats had planned to pay for part of their plan by increasing taxes on high-income earners and corporations. But President Biden acknowledged last week during a CNN town hall that the idea appeared increasingly out of reach.

"I don't think we're going to be able to get the vote," he said.