"If you write a best-selling book, you can be a millionaire, too", he quipped to The Times.
"April 15 is coming", Sanders told the The New York Times.
Pressure has mounted on Sanders again, now that congressional Democrats have pushed for the release of six years of returns by Trump. Bernie Sanders (I-Vt.) on Monday, April 15, from 6:30 to 7:30pm ET in Bethlehem, Pennsylvania.
"On the day in the very immediate future, certainly before April 15, we release ours, I hope that Donald Trump will do exactly the same". He trails only former Vice President Joe Biden, who has yet to officially decide on his candidacy.
Sanders' refusal to release his full tax returns was a relatively minor issue in the 2016 Democratic primaries, when Hillary Clinton goaded him to be more transparent.
Sanders had been promising to release a decade's worth of tax returns for more than a month.
The Marquette University Law School survey released Wednesday showed that among 12 current or potential Democratic presidential candidates, Sanders is a top choice of 32% of voters who don't lean Republican, the former vice president is a top choice of 29%.
Sen. Amy Klobuchar of Minnesota: 8%. Senators Elizabeth Warren of MA and Kirsten Gillibrand of NY have both released a decade of their own returns.
At the start of the month, the Democratic chair of the House Ways and Means Committee requested Trump release his tax returns, using an arcane law. He published another book, Where We Go From Here, in 2018.