The campaign of self-described Democratic socialist Sen. Bernie Sanders, I-Vt., released 10 years of his tax returns Monday. It showed, among other things, that the adjusted gross income of Sanders, who has been outspoken about economic inequality, was $561,293 in 2018, his campaign said in a statement.

The release also confirmed that Sanders’ income crossed the $1 million threshold in 2016 and 2017, though he reported less earnings in his most recent return.

Sanders, a 2020 presidential candidate, said: “These tax returns show that our family has been fortunate. I am very grateful for that, as I grew up in a family that lived paycheck to paycheck and I know the stress of economic insecurity.”

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His 2018 return revealed that he and his wife, Jane, earned over $550,000, including $133,000 in income from his Senate salary and $391,000 in sales of his book, “Our Revolution: A Future to Believe In.”

Sanders’ campaign said in the news release he paid a 26 percent effective tax rate in 2018.

Sanders and his wife disclosed $36,300 in charitable contributions in 2017, but their return did not detail each individual contribution. That same year, the couple announced publicly that they had donated $25,000 as a grant to launch the Sanders Institute, a nonprofit educational organization aligned with the senator’s political and ideological interests.

During his first presidential bid, Sanders released just one year of his tax returns despite primary rival Hillary Clinton pushing him to follow her lead and release multiple years of tax information. He declined to do so, disclosing only his tax return for 2014.

Tax transparency has been in the spotlight as President Trump has bucked decades of presidential tradition by declining to show voters his tax filings and House Democrats have sought to force him to turn them over.

The filings showed that Sanders has been among the top 1 percent of earners in the U.S. According to the liberal-leaning Economic Policy Institute, families in the U.S. earning $421,926 or more a year are part of this group.

During his 2016 presidential campaign and beyond, Sanders declared that wealth inequality was “the great moral issue of our time.”

Now that he’s running for president again, the 77-year-old has continued to advocate for the poor and middle class, including appealing for a $15-an-hour minimum wage and tuition-free colleges and universities.

Back in 2016, Sanders reportedly was the 19th-poorest U.S. senator. However, he has since published four books, and that's said to have helped make him a millionaire.

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"I wrote a best-selling book," Sanders said recently. "If you write a best-selling book, you can be a millionaire, too."

According to estimates, Sanders’ net worth is based on his book sales, royalties and speaking engagements.

The Associated Press contributed to this report.