Democratic presidential hopeful Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) says in a new interview that he will release his tax returns by Monday while acknowledging that he is a millionaire because of book sales.

“On the day in the very immediate future, certainly before April 15, we release ours, I hope that Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE will do exactly the same,” he told The New York Times for a story published Tuesday.

“We are going to release 10 years of our tax returns, and we hope that on that day Donald Trump will do the same,” he continued.

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Sanders acknowledged in the interview that he is a millionaire, attributing it to his book sales. The newspaper noted that his most recent book, “Where We Go From Here,” was published last year.

“I wrote a best-selling book,” he told the Times. “If you write a best-selling book, you can be a millionaire, too.”

Sanders and other Democratic presidential candidates have sought to release their own financial information in an effort to push Trump to release his tax returns.

During the 2016 election, Trump cited an IRS audit as his reason for not releasing his returns. The agency has said audits don't prohibit individuals from releasing information about their own finances.

Sanders has long promised to release his tax returns in connection with his presidential campaign and has said he and his wife, Jane, prepare them without an accountant, according to the Times.

Several other Democratic contenders for president, including Sens. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (N.Y.), Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (Minn.) and Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (Mass.) and Washington Gov. Jay Inslee Jay Robert InsleeBarr asked prosecutors to explore charging Seattle mayor over protest zone: report Bottom line Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE, have released years of their tax returns.

Sanders has consistently ranked among the least wealthy senators but has made more money since his 2016 presidential campaign. According to his 2017 Senate financial disclosure forms, he earned roughly $1.06 million that year, most of it from book royalties, according to the Times.