Sen. Bernard Sanders had a frank assessment when asked about the cost of his far-reaching economic agenda, which includes universal health care and free college.
"You don't know. Nobody knows. This is impossible to predict," the Vermont senator told CBS News.
Mr. Sanders and other Democrats running for the 2020 presidential nomination say new taxes on the rich would go a long way toward funding their costly policy proposals. But there is serious doubt the math would add up without inflating the deficit.
It is common for campaigns to put out rosy or optimistic budget estimates for their proposals, just as the White House routinely does when projecting how its budget blueprints will affect the economy.
"We should all just take them with a bit of a grain of salt," said Robert Bixby, executive director of the Concord Coalition, a fiscal watchdog group.
Mr. Sanders and his allies have been unapologetic in advocating for his pricey policy agenda, which estimates have pegged at costing anywhere between $60 trillion and $100 trillion over 10 years.
Rep. Alexandria Ocasio-Cortez pushed back against budget hawks warning against the eye-popping costs while campaigning for Mr. Sanders in Iowa City over the weekend, saying Pentagon advocates don't bat an eye at the costs of President Trump's Space Force, for example.
"So there is a whole lot of math that is not adding up on their end in any way shape or form because I have said it over and over and over again: They only ask how do we pay for it when it comes to educating our children and making sure our health care is covered and making sure the people actually enjoy public systems and saving our planet," the New York Democrat said.
Former South Bend, Indiana, Mayor Pete Buttigieg stands apart from his 2020 Democratic rivals in saying his party needs to embrace the issue of deficits as Republicans have lost their way on that cause.
"Even though I'm the youngest person running for president, I'm old enough to remember when Republicans talked about the deficit all the time, only for this president to have a trillion-dollar deficit on his watch, mostly as a consequence of these giant tax cuts for corporations and the wealthy," Mr. Buttigieg said at a Fox News town hall over the weekend.
He is the only major Democratic presidential candidate whose health care plan doesn't blow a hole in the federal budget, according to a recent analysis by a nonpartisan watchdog group.
Under a "central" estimate, his plan would save $450 billion over 10 years, according to the Committee for a Responsible Federal Budget.
Randy Roos of Sioux City, Iowa, said he is backing Mr. Buttigieg partly because he thinks the former mayor is being more candid about his plans than some of his far-left rivals.
"He's been straight-up, he's not blowing smoke up anybody's rear end," the 68-year-old said.
Under the same scenario, meanwhile, Mr. Sanders' "Medicare for All" plan would add $13.4 trillion to the deficit, Sen. Elizabeth Warren's proposal would add $6.1 trillion, and former Vice President Joseph R. Biden's would add $800 billion.
Part of the issue is that Ms. Warren and Mr. Sanders are being highly optimistic about the amount of money they plan to bring in with new taxes on the wealthy, said Brian Riedl, a senior fellow at the Manhattan Institute, a think tank focused on free-market ideas.
"The idea that billionaires are going to sit idly by and pay this exorbitant new tax without hiring tax attorneys to help them move the money around is somewhat ludicrous," he said.
Mr. Riedl has estimated that Mr. Sanders' policy proposals would add up to as much as $97.5 trillion in new federal spending, only a small fraction of which would be offset by new taxes.
Mr. Sanders has proposed a 1% tax on net worth above $32 million that would rise to 8% on wealth above $10 billion.
It would raise between $2.8 trillion and $3.3 trillion over 10 years, according to recent projections from the University of Pennsylvania's Penn Wharton Budget Model, well short of the $4.35 trillion the Sanders campaign has estimated.
Ms. Warren's tax on the super-wealthy would generate between $2.3 trillion and $2.7 trillion, according to an estimate from the Penn Wharton model, also short of the campaign's $3.75 trillion projection.
The Warren campaign said in response that leading independent experts have concluded that the senator's plans are "fully paid for."
"Her offsets, like the wealth tax, will produce trillions in revenue to fund middle-class investments that will grow our economy and provide families with more financial security," said campaign spokeswoman Saloni Sharma.
Mr. Bixby said he would be "skeptical" of the numbers behind some of the proposed revenue increases proponents have rolled out.
"It's a very tempting thing when you're trying to finance a big new spending agenda to, in effect, say well, we're going to tax the rich because most people say well, that doesn't affect me," he said. "Then you start talking about people at the upper-income end and they'll start shifting money around so it's not taxed."
Indeed, the campaigns could be underestimating the avoidance, or "leakage," rate of people who find workarounds, said Janet Holtzblatt, a senior fellow at the Urban-Brookings Tax Policy Center.
"I would say they were optimistic," Ms. Holtzblatt said.
She said over time the candidates could raise a substantial amount of money and the proposals could work.
"But it's not going to happen on Day One," she said. "It's going to take time to get the infrastructure in place, and even then there will be leakage."
Mr. Biden has not proposed a wealth tax, but the Penn Wharton model estimated his tax proposals would generate between $2.3 trillion and $2.6 trillion in revenue over a decade, short of the $3.2 trillion the campaign has estimated.
• Seth McLaughlin contributed to this report from Iowa.