Headed into the November election — and with a big fight over expiring tax cuts looming in 2025 — Democrats are pushing once again to raise taxes on the wealthiest Americans.

Earlier this month, President Biden unveiled a budget plan that proposes to raise more than $4.5 trillion in new taxes over the next decade, largely by targeting corporations. While repeating his pledge that families making less than $400,000 a year should not pay more tax, he revived and expanded upon an idea that Democrats have been kicking around for more than a decade: A minimum tax on the ultrawealthy.

Biden proposes to raise $503 billion over the next decade by imposing a 25 percent tax on people who claim more than $100 million in assets — a source of wealth that has long been beyond the reach of the Internal Revenue Service. The new levy would be assessed not just on annual income but on the annual increase in the value of their holdings, including stocks and real estate, even if that additional value was not realized because those assets were not sold.

“I’m a capitalist. If you want to make or can make a million or millions of bucks, that’s great. Just pay your fair share in taxes,” Biden said in his State of the Union address this month. “No billionaire should pay a lower federal tax rate than a teacher, a sanitation worker or a nurse.”

Many independent tax experts say such a levy would be almost impossible to enforce: The IRS would struggle to assess anyone’s total net worth, much less the complex fortunes of the ultrawealthy, they say.

“There’s never going to be a comprehensive database at the IRS of everybody’s assets,” said Garrett Watson, a researcher at the right-leaning Tax Foundation, who also cautioned that trying to tax assets as volatile as stocks would create a highly “unstable” source of federal revenue.

Moreover, any attempt to value assets would lead to billionaires finding new ways to hide those assets, said Eric Zwick, an economist and tax expert who teaches finance at the University of Chicago. Much simpler changes to the tax code could raise revenue from rich people without forcing the IRS to estimate unrealized gains, Zwick said, including reducing the deduction for business income claimed on personal tax returns — a tax break created in Trump’s 2017 tax cuts.

That change would be “pretty straightforward and it doesn’t require us to define a new concept of income,” Zwick said, adding: “My instinct on [Biden’s proposal] is the economic benefits are relatively low — but it has political appeal.”

At least since billionaire investor Warren Buffett complained in 2011 that he pays a lower tax rate than his secretary, Democrats have been looking for a way to rectify the imbalance. There may be momentum on the global level as well. After reaching a historic agreement on a minimum tax rate for multinational corporations in 2021, thanks in part to the Biden administration’s push, finance ministers used an international meeting of the G-20 group of nations last month to debate creating a minimum personal tax for all of the world’s 3,000 billionaires.

“There has been a growing movement over the last 10 years [that recognizes] we need to raise marginal tax rates on the wealthy. But if you don’t also tax unrealized gains, you’re not really getting at the source of the problem,” said David Kass, an analyst with the left-leaning organization Americans for Tax Fairness, adding that Democrats increasingly see taxing the rich as a winning campaign issue.

Democrats’ effort to target billionaires is based on the assumption that simply raising tax rates on conventional income will fail to increase revenue collected from the richest people.

“The entire tax code became this giant block of Swiss cheese that any smart tax accountant can navigate through on behalf of the richest people and the biggest corporations,” said Sen. Elizabeth Warren (D-Mass.). “It’s not that we can’t figure out what a marginal tax is or what a progressive tax structure looks like. The problem is all the exceptions baked in the code.”

Warren and others on the left want to change the definition of income. They point to very wealthy people, with almost no wage income, who live off their investments by obtaining tax-free loans before passing on tax-advantaged appreciated assets to heirs. This strategy — called “buy, borrow, die” — was detailed in a 2021 ProPublica investigation that helped focus fresh attention on the idea of taxing wealth rather than income.

Senate Budget Committee Chair Sheldon Whitehouse (D-R.I.) said he thinks it would be worth the IRS’s time to conduct the type of complicated investigations needed to collect a wealth tax. “While each [high-net-worth taxpayer] may take considerable effort to figure out what their unrealized gains are, the tax payments for doing that make it easily worthwhile,” he said.

Biden often claims that billionaires pay an effective tax rate of just 8 percent. But that number is based on defining rich people’s “income” very broadly to include the rising value of their assets, not just the money they earn in wages or take in capital gains. People in the top 1 percent of the income distribution do pay on average more than 20 percent in income taxes and more than 30 percent in all federal taxes, according to Treasury calculations.

The issue has also come before the Supreme Court, which heard a case this term brought by activists seeking to preemptively prevent Congress from taxing unrealized gains. They argued that imposing such a tax on holdings whose value the owner hasn’t received yet is unconstitutional. The high court is expected to rule on the case in the spring.

While Democrats have long sought to target the rich with new taxes, they have often run into strong opposition — and not just from Republicans.

As president, Barack Obama pushed without success for the “Buffett rule,” which would have required Americans to pay a tax rate of at least 30 percent on annual income over $1 million. Even when Democrats have controlled both chambers of Congress, they have had to scale back their ambitions on taxes. Two years ago, for example, Democrats failed to end the “stepped-up basis” that lets families avoid taxes on appreciated holdings that they pass on to their heirs. Resistance from centrist Democrats such as Sen. Joe Manchin III (W.Va.) proved a powerful curb.

Rates for the wealthy were once much higher than they are today. When President Franklin D. Roosevelt took office in 1933, for instance, he boosted the top tax rate from 25 percent to 63 percent. The top rate soared again, to more than 90 percent, during World War II. And when President Ronald Reagan took office in 1981, a married couple was still paying a top marginal tax rate of 70 percent on every dollar earned over $215,400 — the equivalent of $730,000 today.

Reagan pushed to cut the top rate to 50 percent during his first year as president and to 28 percent in his second term. Since then, the top rate has moved in a fairly narrow band and currently stands at 37 percent for a couple with income above $693,750.

Meanwhile, Republican opposition to higher taxes of any kind remains staunch. Last week, Rep. Lisa C. McClain (R-Mich.) argued with White House budget director Shalanda Young about how much the wealthy should pay, noting that the top 1 percent of earners paid 42 percent of all income taxes in 2020.

“What is the number that the wealthy and the corporations would have to pay for you and this administration to feel like they paid their fair share?” McClain said. “We should be celebrating those people.”

For their part, Democrats have redefined what it means to be rich. Obama campaigned on a promise not to raise taxes on the middle class, which he defined as couples making less than $250,000 a year. But during the “fiscal cliff” negotiations in 2012, he and Biden, his vice president at the time, adjusted that definition to include couples making up to $450,000, a standard Biden has more or less adopted.

“The Republicans don’t want to tax anybody, and the Democrats don’t want to tax 98 percent of anybody,” said Michael Graetz, emeritus professor of tax law at Columbia University. “That’s where we are.”