Former vice president Joe Biden, still the favorite to become the Democratic Party’s nominee for president, and his wife, Jill, earned $4.6 million in 2018. The Bidens also reported earning $11 million in 2017, the year after he left office.

This means “Middle-Class Joe” earned a respectable $15 million in the two years since leaving the White House in 2016. The fact their earnings fell 57 percent in 2018 means the Bidens are somewhere in the bottom half of the list of more than 11 million American millionaires.

Millionaires, who are described as persons with at least $1 million in investable assets, comprise three percent of the U.S. population. The United States has a population of more than 237 million persons.

Biden admitted their combined wealth was derived largely from his six-figure speaking engagements and a lucrative deal to write a set of books. Biden has given dozens of paid speeches since leaving office and was paid as high as $234,820 for a speech.

In 2017, he and his wife signed a three-book deal with Flatiron Books reported to be worth $8 million. In addition, the Bidens reported current assets worth between $2.2 million and $8 million. This compares to $303,000 to just over $1 million he reported in January 2017. Biden drew a salary of $243,500 per year as vice president.

Apart from the speaking and book tours, Biden earns money from his salary as a paid professor at the University of Pennsylvania.

The combined wealth of the Bidens is larger than any of his major Democratic primary opponents. It's also raising eyebrows despite his three closest rivals as presidential standard bearer -- Sen. Kamala Harris (D-CA), Sen. Bernie Sanders (I-VT) and Sen. Elizabeth Warren (D-MA) -- also being millionaires .

The wealth of the Democrats running for president, however, pales when compared to president Donald Trump, whose wealth was estimated at $3.1 billion by Forbes. None of the 23 declared Democrat candidates for president are billionaires.

As might be expected the Bidens’ income increased after Joe Biden left office. The Bidens reported an adjusted gross income of $396,456 in their 2016 tax returns.

Their combined income included Biden’s annual salary as vice president and his government pension, and Mrs. Biden’s income as a community college professor plus social security benefits.

The Biden’s income boost has also been accompanied by a heavier tax burden. The Bidens’ paid an effective federal tax rate of 33 percent compared to the 24 percent they paid in 2016. They reported charitable donations of $5,889 in 2016, $1 million in 2017 and $276,000 in 2018.