As a self-described Democratic socialist, Sen. Bernie Sanders, I-Vt., has been outspoken about economic inequality.

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

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

Sanders will join Fox News Channel for a Town Hall co-anchored by Bret Baier and Martha MacCallum on Monday, April 15, at 6:30 p.m. ET in Bethlehem, Pennsylvania.

BERNIE SANDERS FAST FACTS: 5 THINGS TO KNOW ABOUT THE VERMONT SENATOR

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

"I wrote a best-selling book," Sanders told The New York Times. "If you write a best-selling book, you can be a millionaire, too."

According to estimates from Celebrity Net Worth, Sanders’ net worth is roughly $2 million, based on his book sales, royalties and speaking engagements.

After receiving pressure to release his tax returns during his presidential bid, Sanders released 10 years worth of returns on Monday, April 15, which is also tax day.

According to those returns, the Vermont senator's adjusted gross income was $561,293 in 2018 and paid a 26 percent effective tax rate. In 2017, his adjusted gross income was $1,131,925.

“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 upon the release of his returns.

His returns also showed that Sanders only gave 3.4 percent of his adjusted gross income in 2018 -- though his statement said his returns don't account for the fact that all the proceeds from his book "The Speech" go directly to charity.

Here’s a look at what else is known about the senator’s income and assets.

Income as a senator

As a senator, Sanders makes $174,000 a year, according to the Congressional Research Service, which reported on Senate salaries last year.

Book royalties

Sanders made $880,091.14 in book royalties in 2017, according to the senator’s financial disclosure documents filed in May 2018.

Those royalties included a $505,000 advance on his book “Where We Go From Here,” which was published in 2018, Newsweek reported.

Sanders has three books published by Macmillan and one book, “Outside in the White House,” published by Verso Publications.

Album royalties

In 1987, Sanders recorded a politically focused folk album through Todd R. Lockwood Works called “We Shall Overcome.”

WHAT IS 'DEMOCRATIC SOCIALISM'? BERNIE SANDERS' POLITICAL IDEOLOGY EXPLAINED

During his 2016 campaign, the album got some attention -- though not always positive, The Guardian reported.

However, he did make some royalties on the resurfaced music. In his 2016 financial disclosure documents, Sanders reported receiving $2,520.60, but in 2017 he only received $539.47.

Real estate

In 2015, Politico reported, Sanders owned at least two homes -- one in Vermont and one in Washington, D.C.

According to Forbes, Sanders and his wife, Jane O’Meara Sanders, bought a four-bedroom, two-and-a-half-bathroom house in Vermont in 2009 for $405,000. He also reportedly bought a one-bedroom town house in D.C. in 2007 for $488,999.

Sanders bought a third residence in 2016 -- a house on Lake Champlain in Vermont -- for $575,000, according to The Washington Post.

Pension and retirement savings

From 1981 to 1989, Sanders served as the mayor of Burlington, Vt. He receives an annual pension for that service and in 2017, he received $5,137, according to his financial disclosure documents.

According to estimates from Money, Sanders’ retirement savings total more than $1 million.

Other assets

All other assets on Sanders’ 2017 financial disclosure form, including stock and mutual fund accounts, were listed under his spouse, except for two joint accounts -- one with the U.S. Senate Federal Credit Union valued at somewhere between $100,001 and $250,000, and one with the People's United Bank in Vermont -- its value is between $50,001 and $100,000.