Sen. Bernie Sanders has a history of pressuring opponents to release tax returns, without doing the same himself.

Sanders challenged his 2006 Senate opponent to release tax returns and said he'd be "happy" to as well, but there's no record of him following through.

The Vermont senator released just one return during his 2016 campaign, and has still not released any returns since launching his 2020 campaign.

Sanders pledged to release 10 years of returns in late February, but has still not done so and hasn't offered a timeline on when they'll be made public.

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.

52 PHOTOS Average tax refund in every U.S. state See Gallery Average tax refund in every U.S. state Texas Average refund: $3,206 Number of refunds: 10,087,693 Total income tax refunded: $32.3 billion Louisiana Average refund: $3,115 Number of refunds: 1,611,412 Total income tax refunded: $5 billion Connecticut Average refund: $3,099 Number of refunds: 1,396,609 Total income tax refunded: $4.3 billion Oklahoma Average refund: $3,098 Number of refunds: 1,300,577 Total income tax refunded: $4 billion New York Average refund: $3,059 Number of refunds: 7,712,210 Total income tax refunded: $23.6 billion New Jersey Average refund: $3,013 Number of refunds: 3,479,321 Total income tax refunded: $10.5 billion Wyoming Average refund: $2,989 Number of refunds: 214,649 Total income tax refunded: $641.6 million North Dakota Average refund: $2,983 Number of refunds: 277,422 Total income tax refunded: $827.4 million Florida Average refund: $2,933 Number of refunds: 7,854,538 Total income tax refunded: $23 billion Mississippi Average refund: $2,922 Number of refunds: 1,018,429 Total income tax refunded: $2.97 billion California Average refund: $2,911 Number of refunds: 13,594,703 Total income tax refunded: $39.5 billion Washington D.C. Average refund: $2,900 Number of refunds: 277,399 Total income tax refunded: $804.5 million Illinois Average refund: $2,900 Number of refunds: 4,973,653 Total income tax refunded: $14.4 billion Maryland Average refund: $2,861 Number of refunds: 2,329,288 Total income tax refunded: $6.7 billion Massachusetts Average refund: $2,850 Number of refunds: 2,704,250 Total income tax refunded: $7.7 billion Alaska Average refund: $2,843 Number of refunds: 276,887 Total income tax refunded: $787 million Nevada Average refund: $2,830 Number of refunds: 1,111,952 Total income tax refunded: $3 billion Georgia Average refund: $2,832 Number of refunds: 3,606,774 Total income tax refunded: $10.2 billion Alabama Average refund: $2,802 Number of refunds: 1,650,125 Total income tax refunded: $4.6 billion Virginia Average refund: $2,771 Number of refunds: 3,129,030 Total income tax refunded: $8.7 billion Arkansas Average refund: $2,759 Number of refunds: 989,288 Total income tax refunded: $2.7 billion Tennessee Average refund: $2,726 Number of refunds: 2,465,816 Total income tax refunded: $6.7 billion Utah Average refund: $2,681 Number of refunds: 1,033,141 Total income tax refunded: $2.8 billion Washington Average refund: $2,681 Number of refunds: 2,749,362 Total income tax refunded: $7.4 billion Arizona Average refund: $2,672 Number of refunds: 2,244,925 Total income tax refunded: $6 billion Kansas Average refund: $2,665 Number of refunds: 1,044,275 Total income tax refunded: $2.8 billion New Mexico Average refund: $2,657 Number of refunds: 724,549 Total income tax refunded: $1.9 billion South Dakota Average refund: $2,651 Number of refunds: 321,372 Total income tax refunded: $852 million West Virginia Average refund: $2,649 Number of refunds: 649,049 Total income tax refunded: $1.7 billion Kentucky Average refund: $2,648 Number of refunds: 1,590,274 Total income tax refunded: $4.2 billion Delaware Average refund: $2,648 Number of refunds: 365,749 Total income tax refunded: $968.4 million Rhode Island Average refund: $2,643 Number of refunds: 436,490 Total income tax refunded: $1.1 billion Pennsylvania Average refund: $2,643 Number of refunds: 5,071,264 Total income tax refunded: $13.4 billion Colorado Average refund: $2,636 Number of refunds: 2,014,233 Total income tax refunded: $5.3 billion North Carolina Average refund: $2,629 Number of refunds: 3,580,471 Total income tax refunded: $9.4 billion Nebraska Average refund: $2,615 Number of refunds: 711,103 Total income tax refunded: $1.8 billion Indiana Average refund: $2,612 Number of refunds: 2,577,994 Total income tax refunded: $6.7 billion Iowa Average refund: $2,602 Number of refunds: 1,141,151 Total income tax refunded: $3 billion New Hampshire Average refund: $2,602 Number of refunds: 558,359 Total income tax refunded: $1.4 billion Missouri Average refund: $2,601 Number of refunds: 2,220,029 Total income tax refunded: $5.7 billion South Carolina Average refund: $2,569 Number of refunds: 1,719,299 Total income tax refunded: $4.4 billion Hawaii Average refund: $2,564 Number of refunds: 535,763 Total income tax refunded: $1.4 billion Michigan Average refund: $2,560 Number of refunds: 3,776,668 Total income tax refunded: $9.7 billion Ohio Average refund: $2,517 Number of refunds: 4,570,589 Total income tax refunded: $11.5 billion Minnesota Average refund: $2,516 Number of refunds: 2,112,212 Total income tax refunded: $5.3 billion Idaho Average refund: $2,457 Number of refunds: 561,133 Total income tax refunded: $1.4 billion Wisconsin Average refund: $2,436 Number of refunds: 2,236,886 Total income tax refunded: $5.4 billion Montana Average refund: $2,401 Number of refunds: 372,817 Total income tax refunded: $895 million Oregon Average refund: $2,398 Number of refunds: 1,431,924 Total income tax refunded: $3.4 billion Vermont Average refund: $2,392 Number of refunds: 254,192 Total income tax refunded: $608 million Maine Average refund: $2,336 Number of refunds: 509,896 Total income tax refunded: $1.2 billion Average tax refund by state Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

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.

42 PHOTOS States where Americans pay the highest in state income taxes See Gallery States where Americans pay the highest in state income taxes California State income tax: 1% to 13.3% Maine State income tax: 5.8% to 10.15% Oregon State income tax: 5% to 9.9% Minnesota State income tax: 5.35% to 9.85% Iowa State income tax: 0.36% to 8.98% New Jersey State income tax: 1.4% to 8.97% Vermont State income tax: 3.55% to 8.95% Washington, DC State income tax: 4% to 8.95% New York State income tax: 4% to 8.82% Hawaii State income tax: 1.4% to 8.25% Wisconsin State income tax: 4% to 7.65% Idaho State income tax: 1.6% to 7.4% South Carolina State income tax: 0% to 7% Connecticut State income tax: 3% to 6.99% Arkansas State income tax: 0.9% to 6.9% Montana State income tax: 1% to 6.9% Nebraska State income tax: 2.46% to 6.84% Delaware State income tax: 2.2% to 6.6% West Virginia State income tax: 3% to 6.5% Georgia State income tax: 1% to 6% Kentucky State income tax: 2% to 6% Louisiana State income tax: 2% to 6% Missouri State income tax: 1.5% to 6% Rhode Island State income tax: 3.75% to 5.99% Maryland State income tax: 2% to 5.75% North Carolina State income tax: 5.75% Virginia State income tax: 2% to 5.75% Oklahoma State income tax: 0.5% to 5.25% Massachusetts State income tax: 5.1% Alabama State income tax: 2% to 5% Mississippi State income tax: 3% to 5% Utah State income tax: 5% Ohio State income tax: 0.495% to 4.997% New Mexico State income tax: 1.7% to 4.9% Colorado State income tax: 4.63% Kansas State income tax: 2.7% to 4.6% Arizona State income tax: 2.59% to 4.54% Michigan State income tax: 4.25% Illinois State income tax: 3.75% Indiana State income tax: 3.3% Pennsylvania State income tax: 3.07% North Dakota State income tax: 1.1% to 2.9% Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

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."

11 PHOTOS Bernie Sanders and wife, Jane See Gallery Bernie Sanders and wife, Jane Former Democratic U.S. presidential candidate Senator Bernie Sanders hugs his wife Jane after making a motion to suspend the rules and nominate Hillary Clinton as the Demcoratic presidential nominee at the Democratic National Convention in Philadelphia, Pennsylvania, U.S. July 26, 2016. REUTERS/Carlos Barria U.S. Democratic presidential candidate Bernie Sanders gets a kiss from his wife Jane as he addresses supporters following the closing of the polls in the California presidential primary in Santa Monica, California, U.S., June 7, 2016. REUTERS/Lucy Nicholson U.S. Democratic presidential candidate Bernie Sanders and his wife Jane wave to the audience during a rally in Vallejo, California, May 18, 2016. REUTERS/Stephen Lam PORTSMOUTH, NH - With Jane Sanders, Democratic Presumptive Nominee for President former Secretary of State Hillary Clinton attends a rally with Senator Bernie Sanders (I-VT) at Portsmouth High School Gymnasium in Portsmouth, New Hampshire on Tuesday, July 12, 2016. (Photo by Melina Mara/The Washington Post via Getty Images) Jane O'Meara Sanders walks on the floor during the third day of the Democratic National Convention at the Wells Fargo Center, July 27, 2016 in Philadelphia, Pennsylvania. / AFP / Robyn BECK (Photo credit should read ROBYN BECK/AFP/Getty Images) Democratic U.S. presidential candidate Bernie Sanders kisses his wife, Jane O'Meara Sanders, at a rally in Hollywood, Los Angeles, California, United States October 14, 2015. REUTERS/Lucy Nicholson U.S. Democratic presidential candidate Bernie Sanders (C) hugs his wife Jane Sanders (L) while actress Susan Sarandon surveys the overflow room at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich U.S. Democratic presidential candidate Bernie Sanders' wife Jane (R) waves to the crowd as Sanders acknowledges her and his step daughters Carina (L) and Heather (C) as Sanders addresses his final campaign rally before the Iowa Caucus at Grand View University in Des Moines, Iowa January 31, 2016. REUTERS/Jim Bourg Jane Sanders, wife of Vermont Senator and 2016 Democratic presidential candidate Bernie Sanders, listens during an interview following a campaign event in Fort Madison, Iowa, U.S., on Friday, Jan. 29, 2016. In advance of Monday's Iowa caucuses, the first electoral contest of the presidential primaries, Jane Sanders has ventured out often on her own, sometimes with multiple events the same day. Photographer: T.J. Kirkpatrick/Bloomberg via Getty Images Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, walks with his wife Jane Sanders ahead of the Jefferson-Jackson Dinner in Des Moines, Iowa, U.S., on Saturday, Oct. 24, 2015. With Vice President Joe Biden officially out of the presidential race, the nation's first nominating contest between front-runner Hillary Clinton and Sanders is gaining steam, according to a new Bloomberg Politics/Des Moines Register Iowa Poll. Photographer: Daniel Acker/Bloomberg via Getty Images FLORENCE, SOUTH CAROLINA - SEPTEMBER 12: With his wife Jane O'Meara, Vermont Senator Bernie Sanders (I-VT) prepares to speak at a Florence Town Hall Meeting in an arena in Florence, South Carolina on Saturday September 12, 2015. (Photo by Melina Mara/The Washington Post via Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

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.

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