(CNN) Bernie Sanders is (finally) going to release his tax returns. But questions over how he handles his wealth in the context of his campaign for the 2020 Democratic presidential nomination remain. And the early returns should be concerning to Sanders' backers.

In an interview with The New York Times on Tuesday , Sanders said that "April 15 is coming," adding, "We wanted to release 10 years of tax returns. April 15, 2019, will be the 10th year, so I think you will see them." (Sanders, despite effectively running for president without pause since 2015, has released only a single year's tax return, for 2014 .)

Reminded by the Times reporter that he is now someone of considerable means, Sanders retorted: "I wrote a best-selling book. If you write a best-selling book, you can be a millionaire, too."

Uh, what?

Sanders got a pass on some of his acerbic-ness during the 2016 presidential campaign because a) most people didn't think he could win and b) others found it part of his cantankerous but righteous personality. But let's think about that campaign for another minute. Close your eyes and imagine if Hillary Clinton, who beat Sanders for the Democratic nomination, when asked about the extent of her wealth, said "I wrote a best-selling book. If you write a best-selling book, you can be a millionaire, too."

She would have been pilloried! People would have blasted Clinton for being out of touch with the average person who, obvi, doesn't have the opportunity to write a book -- best-selling or not. Telling the average middle-class American that they too can be rich if they only write a best-selling book is like telling them that if they would just star in a movie then they could be a big star, too. Yes, it's true. No, for almost everyone, it's not realistic.

In 2016, Clinton was the frontrunner -- and most of the attention and energy in the political media was focused on her. But now Sanders is one of the frontrunners. Which means that he simply cannot be as dismissive, curt or out-of-touch as his "write a best-selling book" comments come off.

Then there is the broader problem of Sanders' wealth as it relates to his core campaign message that "millionaires and billionaires" have cornered far too much wealth and power in this country -- and need to be reined in by a more activist federal government. Now that Sanders is one of those "millionaires and billionaires," it could complicate that message -- and his appeal as the underfunded outsider taking on the monied interested in Washington.

Faiz Shakir, Sanders' campaign manager, told CNN on Tuesday night that the senator "believes in opportunity for all, and the fact that he is somebody who has personally benefited from that opportunity is something that he feels should be a shared opportunity with everyone else. He's made some money off a book. And I think that the opportunity that he has had is evaporating for so many others. He feels that strongly."

Sanders, talking to the Times, tried to draw a line between himself and the likes of President Donald Trump, a self-described billionaire. "Not being a billionaire, not having investments in Saudi Arabia, wherever he has investments, all over the world, mine will be a little bit more boring," Sanders said of his returns when compared to those of Trump's. (The President has never released any of his tax returns and pledged again Wednesday morning that he is under no obligation to do so.)

On the most practical level, Sanders is right. He isn't as rich as Trump. There's a big difference between a millionaire and a billionaire. And there's no reason that someone who is wealthy can't speak powerfully about the economic plight of the less fortunate and the deeply negative impact the wealthiest of Americans are having on society.

But this is politics. And the combination of Sanders now being a member of the economic strata he has long derided and his totally tone-deaf answer on how he got there spells at least the potential for trouble down the line for the Vermont senator.

CORRECTION: This story has been updated to correctly state that Sanders released his full 2014 tax return during the 2016 campaign.