Bernie Sanders often demands his opponents release their tax returns\, but his own record of releasing them isn\'t stellar

Sen. Bernie Sanders has built a reputation for being consistent in his roughly 40-year career in politics.

When voters who back him are asked why they support Sanders, they often cite the fact he's stuck to the same message for decades. Simply put, those who support Sanders seem to feel that they know what they're going to get with the Vermont senator.

But there seems to be one gap in this trend of consistency for Sanders: releasing tax returns.

Sanders has a history of calling out his political opponents to release their returns, with a flimsy record of doing the same.

During his 2006 Senate race, Sanders made a big deal of his Republican opponent releasing his returns during a debate and pledged to do the same. At the time, his campaign said it would be a "good idea" for candidates to release their federal and state returns.

"Are you prepared to release your taxes?" Sanders said to his GOP opponent, then-Republican Senate nominee Richard Tarrant, during a debate in Burlington, Vermont. "Anytime you want to release your taxes, Mr. Tarrant, which you originally said that you would, but I believe changed your mind ... we are happy to do that."

There's no record of Sanders ever following through on this.

When he ran for president 2016, Sanders was criticized for not being more transparent with his tax returns.

"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 said in an April 2016 interview.

He ultimately released his 2014 return, which appears to be the only one the senator has made public.

Sanders is among the least affluent lawmakers Congress, and is the 12th-poorest member in the Senate, according to Roll Call. But book royalties seem to have boosted Sanders' wealth in the past two years or so, and he's estimated to have a net worth of roughly $2 million.

When asked why he didn't release more returns in 2016 during a CNN town hall forum in February, Sanders said, "I didn't end up doing it because we didn't win the nomination."

During the town hall, Sanders pledged to release 10 years of tax returns "soon."

"We have to just do a few more little things," Sanders said at the time, assuring the audience that his returns are "boring."

Over a month later, Sanders has still not released his tax returns or provided a specific timeline on when they'll be released.

"We have it all done and it's just a question of dotting the i's and crossing the t's," Sanders said in an interview on Sunday.

"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."

Trump has been slammed by critics for not releasing tax returns and breaking decades of precedent in the process. House Democrats have zeroed in on this issue, and are working to obtain them.

Several other 2020 Democratic candidates have already released their returns.

Sen. Kirsten Gillibrand of New York, who was the first to do so, said in an interview with CNN in late March said, "I think the American people have a right to know - particularly people who are running for president, and our president today, who has not disclosed his tax returns - have a right to know."

"It allows them to know that you are working for them, and nobody else," Gillibrand added.

Sanders has consistently blasted the influence of money in politics and has refused to accept any corporate PAC campaign donations. He's made overturning Citizens United one of the central causes of his political career and has railed against Wall Street and corporate interests influencing Washington lawmakers.

The democratic socialist is considered a leading contender for the Democratic nomination in 2020, trailing closely behind former Vice President Joe Biden in a number of recent polls.

Meanwhile, Americans do not have a full picture of the Vermont senator's financial history.

Sanders' 2020 campaign has not responded to requests from INSIDER for an updated timeline on when the senator will release his tax returns.