WASHINGTON (Reuters) - U.S. Democratic presidential contender Bernie Sanders released 10 years of tax returns on Monday, providing details of his growing status as a millionaire fueled by a sharp jump in income from book royalties since his losing 2016 White House run.

Sanders, a U.S. senator who routinely rails against the “millionaires and billionaires” he says have rigged the system to protect their wealth and power, had an adjusted gross income of $561,293 in 2018, $1,131,925 in 2017 and $1,062,626 in 2016, the returns showed.

He augmented his Senate salary with book royalties in each of those years, particularly in 2016 and 2017 when he made more than $800,000 each year in royalties. Sanders has published three books since the start of his first White House run, including bestsellers “Our Revolution” and “Where We Go From Here.”

In 2014, the year before he launched his first presidential campaign, Sanders had an adjusted gross income of $205,271.

Sanders had faced mounting pressure to release his taxes, with critics saying the democratic socialist’s millionaire status undercut his populist economic message. He has made no apologies for his financial well-being.

“It comes from a book I wrote, pretty good book,” Sanders told a Fox News Channel town hall on Monday. “If anyone thinks that I should apologize for writing a best-selling book, I’m sorry, I’m not going to do it.”

The interest in presidential contenders and their taxes has jumped since Republican President Donald Trump shattered decades of tradition during the 2016 campaign by refusing to release his returns - a stance he has continued since entering the White House.

FILE PHOTO: Democratic U.S. 2020 presidential candidate Bernie Sanders holds an evening public rally along the waterfront in downtown San Diego, California, U.S., March 22, 2019. REUTERS/Mike Blake

Several in the growing field of Democratic 2020 contenders, including Senators Kirsten Gillibrand, Amy Klobuchar, Kamala Harris and Elizabeth Warren, and Governor Jay Inslee of Washington, have released their 2018 returns in recent weeks.

Beto O’Rourke, the former congressman from Texas, released 10 years of his returns on Monday night. In the most recent year released, 2017, he had an adjusted gross income of $366,455. He promised to release his 2018 return as soon as possible.

Most other Democratic contenders have pledged to release their tax returns soon.

CHIDES TRUMP

But the tax question had become more pressing recently for Sanders, who only released one year of returns during his 2016 campaign, as he moved into a strong early position in polls and fundraising among Democrats seeking the 2020 nomination to challenge Trump.

Noting that Trump watches the Fox News Channel, Sanders took the opportunity to chide the president for his failure to release his returns.

“Hey President Trump, my wife and I just released 10 years - please do the same,” he said during the town hall.

Sanders faced criticism for only releasing his 2014 returns during his 2016 Democratic primary battle with Hillary Clinton, a millionaire whom he often derided for giving paid speeches to Wall Street.

The tax returns released on Monday showed Sanders paid a 26 percent effective tax rate on his adjusted gross income in 2018. His effective tax rates in 2016 and 2017, his other high-earning years, were 35 percent and 30 percent, respectively.

As part of his policy agenda, Sanders has proposed higher taxes on top earners and a big expansion of the estate tax, lowering the threshold where it kicks in to $3.5 million from $11 million, and placing a 77 percent tax rate on the portion of estates worth more than $1 billion.