Bernie Sanders took the stage at a fiery Fox News town hall in Bethlehem, Pennsylvania on Monday, and sparks flew almost immediately, as Sanders defiantly refused to explain why he would not voluntarily pay the massive new 52-percent "wealth tax" that he advocated imposing on the nation's richest individuals.

"We'll get through this together," Sanders said at one point, as tensions flared.

Sanders later admitted outright that "you're going to pay more in taxes" if he became president. Just minutes before the town hall began, Sanders released ten years of his tax returns, which he acknowledged showed that he had been "fortunate" even as he pushed for a more progressive tax system.

According to the returns, Sanders and his wife paid a 26 percent effective tax rate on $561,293 in income, and made more than $1 million in both 2016 and 2017. Sanders donated only $10,600 to charity in 2016 and $36,300 in 2017, the records showed, followed by nearly $19,000 in 2018.

But pressed by anchors Bret Baier and Martha MacCallum as to why he was holding onto his wealth rather than refusing deductions or writing a check to the Treasury Department, Sanders began laughing dismissively and, in an apparent non sequitur, asked why MacCallum didn't donate her salary. (“I didn’t suggest a wealth tax," MacCallum responded.)

"Pfft, come on. I paid the taxes that I owe," Sanders shot back. "And by the way, why don't you get Donald Trump up here and ask him how much he pays in taxes? President Trump watches your network a little bit, right? Hey President Trump, my wife and I just released 10 years. Please do the same."

Asked whether Sanders' success -- and subsequent decision to hold onto his cash -- wasn't an implicit endorsement of the capitalist system he has repeatedly called dysfunctonal, Sanders rejected the notion out of hand.

"When you wrote the book and made the money, isn’t that the definition of capitalism and the American dream?” Baier asked, referring to Sanders' bestselling 2016 memoir "Our Revolution."

"No," Sanders replied flatly, after a pregnant pause. "What we want is a country in which everyone has an opportunity. ... A lot of people don't have a college degree. A lot of people are not United States senators."

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Sanders doubled down on his previous defenses of his wealth, which even some progressives have called hypocritical.

"This year, we had $560,000 in income," Sanders said. "In my and my wife's case, I wrote a pretty good book. It was a bestseller, sold all over the world, and we made money. If anyone thinks I should apologize for writing a bestselling book, I'm sorry, I'm not gonna do it."

On whether he supported abortions that occur up to the moment of birth, Sanders retorted, "I think that happens very, very rarely, and I think this is being made into a political issue. At the end of the day, I think the decision over abortion belongs to a woman and her physician, and not the government."

Sanders also said felons, including rapists and murderers, should be able to vote from prison. But he insisted he was not simply courting more potential Democrat voters.

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The Tax Day town hall took place as Sanders emerged as the fundraising front-runner among Democrats, and sought to further distinguish himself from a crowded field of liberal candidates who have largely embraced his progressive proposals, from a sweeping 'Medicare for All' overhaul to a higher minimum wage and free public college education.

"I think Trump is a dangerous president, but if all we do is focus on him, we lose," Sanders said at the town hall.

Separately, Sanders acknowledged that his proposed Medicare for All health care overhaul -- which has also been embraced by other 2020 Democrat hopefuls, including Kamala Harris and Elizabeth Warren -- would mean that many Americans would "pay more in taxes."

Some estimates put the total costs for the plan over 10 years at more than $32 trillion, and say it would necessitate historic tax hikes.

Sanders began by deflecting when asked by Baier whether he was concerned about the rising national debt, saying it was "ironic" that Republicans weren't instead attacking the president.

"You're talking to the wrong guy," Sanders said. "We pay for what we're proposing, unlike the President of the United States."

Sanders more substantively discussed a plan to impose a "speculaton tax" on Wall Street.

"I am concerned about the debt. That's a legitimate concern," Sanders said. "But we pay for what we are proposing. In terms of Medicare for All, we are paying for that by eliminating as I said before, deductibles and premiums. We are going to save the average American family money."

When Baier polled the audience at the town hall -- which was clearly supportive of Sanders throughout -- most indicated they would support Sanders' health care plan, despite currently having private insurance they would lose.

Sanders also warned that climate change poses an existential threat, citing a recent United Nations report claiming that only 12 years remain to make significant changes in global carbon emissions to avert a climate catastrophe. The United Nations made the same prediction in 1989, falsely warning that the world then faced a 10-year deadline that has come and gone.

On immigration, Sanders said we "don't need to demonize immigrants" and proposed "building proper facilities right on the border" and enacting "comprehensive immigration reform." But he said it was "not a real question" when MacCallum asked about the merits of Trump's proposal to send illegal immigrants to sanctuary cities.

The 77-year-old self-proclaimed 'democratic socialist' — the longest-serving Independent member of Congress in history — has also faced criticisms that he mght be too old to serve as president.

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At the town hall, Sanders acknowledged it was a "fair question," but said to applause there is "too much focus on individuals and not enough focus on the American people and what their needs are."

Over the weekend, Sanders sparred with progressive activist groups that pointed out that he has since largely dropped his criticisms of "millionaires and billionaires," opting instead to single out "billionaires" only.

Earlier Monday afternoon, Sanders previewed some of his messaging by asserting that President Trump's "tax policies" will "raise taxes on millions of people."

In an article entitled "Face it: You (Probably) Got a Tax Cut," the New York Times credited liberal messaging with confusing large swaths of the electorate into thinking that their taxes went up, when in fact most saw significant tax savings under Trump's 2017 tax law.

The town hall marked the Vermont senator's first appearance on Fox News Channel since he agreed to be a guest on Baier's show in December 2018. He also participated in a Fox News Channel town hall back in 2016 alongside his then-competitor Hillary Clinton.

Sanders ended the town hall by thanking Fox News for providing him the opportunity.

"Not everybody thought I should come on this show," Sanders said at one point. "Your network does not have a great deal of respect in my world, but I thought it was important to be here."

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Democratic National Committee (DNC) chair Tom Perez has excluded Fox News from hosting a Democrat primary debate. Some congressional Democrats have called that decision inappropriate and unhelpful, and DNC leadership later said it had no objection to Sanders appearing at a Fox News town hall.

Asked whether he felt that the DNC would seek to tip the scales against Sanders -- as leaked emails showed it did in 2016 -- Sanders was optimistic.

"I think we have come a long way since then. We speak to the DNC every week," Sanders told Baier and MacCallum. "And I think the process will be fair."

Since announcing his presidential bid in February, Sanders has hauled in a whopping $18.2 million in the first 41 days of his campaign. But, although Sanders had a fundraising edge over his rivals, Democrats generally haven't raised as much cash as they'd hoped by this point. Many donors have been sitting on the sidelines to see how the contest unfolds, signaling a drawn-out primary battle ahead.

The campaign among Democrats has come into greater focus as declared White House hopefuls reported their first-quarter fundraising totals. Early glimpses provided by nine of the declared candidates showed that Democrats were raising less money than they had in previous cycles and were coming up short against the campaign bank account Trump has been building.

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Democrats collectively raised about $68 million since January, according to the candidates who have already released their fundraising totals. That's less than the $81 million Democrats raised during the same period in 2007, the last time the party had an open primary, according to data from the Federal Election Commission. And, it paled in comparison with the $30 million Trump raised during the first quarter.

"There is no question that the numbers are not at the level that they were with Barack Obama and Hillary Clinton in 2008 by a long shot," said Tom Nides, a Clinton adviser and longtime fundraiser. "Am I worried? No, I'm not worried. But I'm a little bit concerned."

Fox News' Paulina Dedaj and The Associated Press contributed to this report.