(CNN) Bernie Sanders has effectively been running for president steadily since, at least, 2015. And over all that time -- which now spans two campaigns for president -- we've seen exactly one year of his tax returns.

In the 2016 campaign, Sanders released the two summary pages of his 2014 returns, then later released his entire 2014 return. He said he would release a more robust tax history if he became the Democratic nominee, which, well, didn't happen. By contrast, Hillary Clinton released eight years of returns -- spanning 2007-2014 -- in August 2015. A year later, in the midst of the general election against Donald Trump, Clinton released her 2015 return

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With Sanders now installed as one of the frontrunners for the 2020 nomination -- and his past resistance to releasing a full picture of his tax history -- his ongoing vague statements about when he might release more of his returns are of note.

"Do you know what April 15th is? It's Tax Day," Sanders told CNN's Ted Barrett earlier this week when asked if there was an issue to releasing his taxes. "So, I think we want to make sure we have all of them together and as I said, they will be released soon."

"Soon." Asked what date that meant, Sanders replied: "That's it. Thank you very much."

In an interview with "Daily Show" host Trevor Noah that aired Thursday night, Sanders said much the same thing. Here's the exchange

Sanders: April 15th is coming. That will be the tenth year and we will make them all public very...

Noah: What's all? Just so people know.

Sanders: Ten years. And by the way, I'm delighted to do that, proud to do that. Hey Mister Trump [looks at camera], you do the same thing.

Which, maybe? It's possible that in 10 days' time, Sanders releases a decade -- or so -- of back tax returns and this issue recedes. But there's reason to be a bit suspicious of Sanders' promises that his tax release is imminent. Back in February, at a CNN town hall with Wolf Blitzer , the Vermont senator said he would release his taxes "sooner than later."

That was 39 days ago.

Meanwhile, several of Sanders' competitors for the 2020 Democratic nomination -- Sens. Elizabeth Warren (Mass.), Kirsten Gillibrand (N.Y.) and Amy Klobuchar (Minn.) as well as Washington Gov. Jay Inslee -- have already released their past returns.

Why does this matter? As I said, if Sanders makes good on his pledge-ish and releases 10 years of returns on April 15, it won't. But the ongoing scrutiny of Sanders' long-held resistance to releasing the returns speaks to not only his vaunted status in this race but also the unique challenge of running against Trump.

Unlike in the 2016 campaign where Sanders was regarded as a long-shot to knock off Clinton, he is seen as one of the frontrunners for the nomination this time around. Polling suggests that he and Joe Biden start in a class of their own in the race. Top-tier candidates -- i.e. the people who most observers believe have a legitimate chance of being the party's nominee -- have a higher bar on transparency (and lots of other things) than candidates no one sees as having a reasonable path to the nomination.

That goes double -- or maybe even triple -- when the president is Donald Trump. Trump is the first president to release none of his tax returns -- insisting as recently as Thursday that he is under audit and, therefore, won't be releasing them. House Democrats are seeking to use a 1924 law that allows the chairman of the House Ways and Means Committee to request the President's return, although Trump and his administration seem certain to resist that ask, a move that will likely throw the matter into the courts.

Many Democrats believe that to beat Trump next fall, the party has to pick a nominee who provides a stark contrast to Trump on virtually every issue -- including transparency. And that's simply not where Sanders is at the moment.

"Among the candidates still in the race, Sanders' releases are less extensive than anybody's but Donald Trump," wrote PolitiFact's Louis Jacobson back in April 2016 . "He's only released information for one year, which pales compared to most other recent presidential candidates, and even that year's release only includes a summary page, not the full return."

It's three years later. But those facts remain the same.

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