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.) on Saturday pushed back against criticism he received after saying this week that he is a millionaire.

"I didn't know that it was a crime to write a good book, which turned out to be a best-seller," the 2020 Democratic presidential candidate told a crowd in Gary, Ind.

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"I don’t apologize for writing a book that was No. 3 on The New York Times best-seller [list]," he added but reiterated his belief in a "progressive tax system which demands that the wealthiest people in this country finally start paying their fair share in taxes."

Bernie Sanders speaks in Gary, IN: "I didn't know that it was a crime to write a good book, which turned out to be a best seller." pic.twitter.com/vbfWjE0jCr — The Hill (@thehill) April 13, 2019

Sanders told The New York Times in a story published Tuesday that he became a millionaire because of book sales.

“If you write a best-selling book, you can be a millionaire, too,” he told the newspaper. He also said he would release his tax returns by Monday. Some of his competitors for the Democratic nomination have already released their returns.

Sanders's 2017 Senate financial disclosure forms show he made about $1.06 million that year, most of it from book royalties, the Times reported.

The Vermont Independent has also been criticized for not releasing his tax returns but pledged in the Times that he would release them by Monday.

Sanders has described himself as a democratic socialist and has argued that the wealthy, particularly billionaires, do not pay enough in taxes. He is among more than a dozen people vying for the Democratic party's 2020 nomination.