248 millionaires are calling for Democrats to tax billionaires' income. 'We have a rare opportunity to reform our broken tax code.'

• A group of 248 billionaires signed on to a letter calling for a billionaires' income tax.
• Last week, Senate Democrats proposed taxing billionaires' stock gains - before abandoning it.

Nearly 250 millionaires are calling on Democrats to include a tax on billionaires' income to pay for their social spending package.

Democrats are finally narrowing in on what the final proposal will look like, and are reportedly hoping to vote on the legislation on Tuesday.

The latest framework is just $1.75 trillion, half the price tag of Senate Democrats' initial $3.5 trillion package, and omits several tax hikes originally proposed to help pay for initiatives like childcare, Medicare expansion, and climate measures. Instead, it includes a 5% surtax on Americans who earn over $5 million.

Some millionaires want lawmakers to return to an idea they had last week - a tax on billionaires' net worth gains.

In a letter organized by left-leaning advocacy groups Responsible Wealth, Patriotic Millionaires, and Americans for Tax Fairness, 248 millionaires write that moving forward with a billionaires' tax is a "rare opportunity to reform our broken tax code that has for far too long given the very wealthiest ways to avoid paying their share." They note that just around 700 families would be impacted, and the current system reportedly allows for some billionaires to pay nothing in income taxes.

"I think the pushback is, 'Oh, you know, the rich people might, I don't know, go on strike or something if we raise their taxes, so we have to have an appeasement policy,' and that's never worked," Morris Pearl, the chair of the Patriotic Millionaires, told Insider. "But what we're trying to do is tell our congresspeople that no, actually these progressive policies are good for everyone, including the long run best self-interest of those of us who are very wealthy, too."

The groups were referring to a tax plan from Senate Finance Chair Ron Wyden, who crafted a proposal targeting billionaires' assets. Currently, assets like stocks are taxed only when they're sold - what's called a capital gain. But the richest of the rich in America probably aren't selling off their massive stock portfolios; instead, their main form of income is the value that those assets accrue. Under Wyden's plan, that annual increase in value - called unrealized gains - would be taxed every year.

Wyden's proposal would apply only to people making over $100 million annually, or those who have held $1 billion in assets for three years. An analysis from economist Gabriel Zucman found that the tax could bring in $500 billion, with $275 billion collected from just the 10 richest billionaires. A prior White House analysis found that, when unrealized gains are counted as income, billionaires pay an average of just 8.2% in income taxes.

But that proposal to hike taxes on billionaires is currently out of the plan. It lasted for mere hours on Wednesday, before Sen. Joe Manchin expressed his doubts about it. Shortly afterward, Wyden's House counterpart Richard Neal, the Ways and Means Chairman, said it was "very unlikely" to head to the House if it can't pass in the Senate.

Right now, the tax provisions include a 15% corporate minimum tax and that 5% surtax on American millionaires. The millionaires wrote that they support the surtax proposal, but without the billionaires' income tax, many billionaires will still pay low tax rates.

"What we're missing out on is what used to be called the middle-class - the millions and millions and millions of people who were doing okay, who were buying cars and taking their kids to buy ice cream and going to Disneyland and eventually paying their kid's college tuition. That's what's missing from our country," Pearl said. "We have plenty of billionaires. We don't have a shortage of that."