WASHINGTON — Senator Bernie Sanders, whose $18 million fund-raising haul has solidified his status as a front-runner for the Democratic presidential nomination, said Tuesday that he would release 10 years of tax returns by Tax Day on Monday and acknowledged that he has joined the ranks of the millionaires he has denounced for years.

“April 15 is coming,” Mr. Sanders, whose refusal to release his full past returns has become an issue in the campaign, said in an interview in his office. “We wanted to release 10 years of tax returns. April 15, 2019, will be the 10th year, so I think you will see them.”

Told that he was being compared to President Trump, who has refused to release his tax returns, Mr. Sanders got more specific: “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. We are going to release 10 years of our tax returns, and we hope that on that day Donald Trump will do the same.”

Mr. Sanders’s 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. But Mr. Trump’s refusal to release his — and the subsequent effort by House Democrats to force the release — have raised the issue’s profile. Mr. Sanders has bristled at comparisons between his behavior and the president’s.