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The returns showed Buttigieg, with 26.9 percent, followed by Sen. Bernie Sanders, I-Vt., who had 25.1 percent.
If elected, Buttigieg -- a 38-year-old Washington, D.C., outsider -- would become the first openly gay president and youngest-ever person to become president.
The progressive politician, who has called for "a new generation of leadership" in the U.S., has significantly less in his bank account than his competitors — despite being one of Indiana's top-paid mayors.
In 2018, Buttigieg's mayoral salary was $112,314, according to CNBC. In total, Buttigieg and his middle-school teacher husband, Chasten, reported a total income of $152,643. The pair paid $20,000 in federal taxes — for an approximate tax rate of 13.2 percent, according to his tax returns.
Buttigieg, who was elected mayor of South Bend in 2011 at the age of 29, has reported six-figures salaries over the past few years. According to Forbes, he is worth about $100,000.
Last year, he told CNBC he considers himself middle class.
“Everybody’s talking about the middle of the country like it’s some mysterious place and I think it might make sense to have somebody in the mix who actually lives here,” Buttigieg said. “I actually live in a middle-class lifestyle, in a middle-class neighborhood, in the American Midwest.”
He's also far behind his Democratic rivals in terms of wealth.
Last April, Sanders, too, released 10 years worth of returns, which showed his adjusted gross income in 2017 was $1,131,925 — mainly thanks to royalties from his book "Where We Go From Here." Biden, who earned around $230,000 annually during his time in the White House, is worth about $9 million, per Forbes.
This report contains material from previous FOX Business stories.