Sen. Bernie Sanders pledged in late February to release his tax returns and has still not done so.

As Sanders faced questions on when he'll release his tax returns in an interview on Sunday, the senator said he'll "absolutely" make them public and pressured Trump to do the same. But he didn't say when they'll be released.

Sanders has been ranked among the poorer members of the Senate, but has reportedly boosted his wealth in recent years via book royalties.

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It's been over a month since Sen. Bernie Sanders pledged to release his tax returns and there's no updated timeline on when they'll see the light of day.

Sanders has offered vague answers on when he'll release the returns, even as he pressures President Donald Trump to do the same.

In an interview with Margaret Brennan of CBS's "Face the Nation" on Sunday, the Vermont senator was asked if he will be releasing his tax returns. Brennan noted that Sen. Kirsten Gillibrand — who like Sanders is running for president in 2020 — released her 2018 tax returns earlier on March 27.

"Yeah, we will. I mean, we have it all done and it's just a question of dotting the i's and crossing the t's. Yes, we will, absolutely," Sanders said.

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"And by the way, let me challenge President Trump to do the same," he added. "Trust me, we do not have investments in Russia or Saudi Arabia or anyplace else. Yes, we will be releasing them."

The senator on Thursday once again offered a vague answer on when he'll release his tax returns, but was somewhat more specific in terms of a timeline and suggested they'd be made public after April 15.

"Do you know what April 15 is? It's Tax Day," Sanders told CNN. "So, I think we want to make sure we have all of them together and as I said, they will be released soon."

Sanders was then asked if this meant he'd release his returns on April 15, and the senator wrapped up the conversation.

"That's it. Thank you very much," Sanders said before an aide reportedly stepped in and said the senator had answered the question.

Similarly, in an appearance on "The Daily Show" on Thursday, Sanders said, "April 15 is coming. That will be the 10th year and we will make them all public very shortly."

He added, "I'm delighted to do that, proud to do that. Hey Mr. Trump, you do the same thing."

When CNN's Wolf Blitzer asked Sanders about his tax returns during a town hall forum in late February, the senator did not offer a specific timeline but said they would be released "soon." He pledged to release 10 years of returns.

"They're very boring tax returns," Sanders said.

Sanders faced criticism during his 2016 campaign for not sharing more tax returns; he made his 2014 return public after being pressured to do so by former Secretary of State Hillary Clinton in a presidential primary debate. The return showed Sanders and his wife, Jane Sanders, paid $27,653 in federal taxes on adjusted gross income of $205,271.

Read more: Bernie Sanders has made more than $1.75 million from book royalties since 2016 — here's what we know about his wealth and assets

When asked about releasing his tax returns in April 2016, Sanders offered a similar response to what we heard from him on Sunday.

"You'll get them, yes," he said at the time. "Yeah, look, I don't want to get anybody very excited. They are very boring tax returns. No big money from speeches, no major investments. Unfortunately — unfortunately, I remain one of the poorer members of the United States Senate. And that's what that will show."

Sanders is among the least-wealthy members of Congress, and is the 12th-poorest member in the Senate, according to Roll Call. But despite the low-ranking, the senator's net worth is still estimated to be somewhere around $2 million.

The Vermont senator's 2020 campaign did not immediately respond to a request from INSIDER for an updated timeline on when he'll make his tax returns public.

Several 2020 Democrats have already done so.

Meanwhile, Trump continues to face criticism for refusing to publish his own tax documents. During his 2016 campaign, Trump claimed he could not release the returns due to an ongoing audit. Shortly after he was inaugurated, White House counselor Kellyanne Conway said Trump would not release his returns, contending that the public didn't care.

But polls have repeatedly shown that a majority of Americans would like Trump to release his tax returns.

Trump has broken roughly 40 years of precedent by not making his tax returns public, as virtually every major party presidential nominee has done since the mid-1970s.