WASHINGTON, April 15 (Xinhua) -- U.S. Senator Bernie Sanders, who's making his second bid for the U.S. presidency, released 10 years of tax returns on Monday.

Sanders, 77, had an adjusted gross income of 561,293 U.S. dollars in 2018, 1,131,925 dollars in 2017 and 1,062,626 dollars in 2016, the returns showed.

The Vermont senator earned significant income from his books in 2016 and 2017 when he made more than 800,000 dollars each year in royalties.

Since his first White House run, Sanders has published three books, including bestsellers "Our Revolution" and "Where We Go From Here."

In 2014, the year before the launch of his first presidential campaign, Sanders, a self-described Democratic socialist, had an adjusted gross income of 205,271 dollars.

"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," Sanders said in a statement.

Campaign manager Faiz Shakir said Sanders, as "a strong proponent of transparency," hopes President Donald Trump and all Democratic primary candidates will disclose their tax returns.

Beto O'Rourke, another Democratic presidential candidate, also released 10 years of his tax returns on Monday, with the most recent filing showing that he and his wife earned more than 360,000 dollars in income.

Tax transparency has been in the spotlight over Trump's rejection of a decades-old tradition that presidents and White House hopefuls would make their tax returns public.

House Democrats are seeking to force the president to turn his tax returns over.

Besides Sanders and O'Rourke, a number of Democratic presidential candidates -- including Senators Kirsten Gillibrand of New York, Elizabeth Warren of Massachusetts, Kamala Harris of California and Amy Klobuchar of Minnesota -- have released tax records to varying degrees.