WASHINGTON — Joe Biden’s income soared from less than $400,000 a year while he was vice president to more than $15 million in the two years after he left the Obama White House, a spike in wealth due to sales of his 2017 book and speaking fees that routinely ran more than $100,000 per event.

Biden’s campaign released a financial disclosure form and three years of income tax returns starting with 2016, his last year in office, and running through 2018. He had previously released his returns from earlier years.

The documents showed that he made 47 paid speaking appearances from January 2018 through the end of May 2019, 30 of which were on a book tour, for fees totaling $4.29 million.

Biden is the latest Democrat to release tax returns and other financial information as the party’s candidates seek to draw a sharp contrast with President Trump, who has repeatedly refused to do so.

Nonetheless, his sudden affluence could create some political liabilities for Biden, who is trying to make his blue-collar, middle-class roots central to his bid for the Democratic presidential nomination in 2020. He also frequently mentions in campaign speeches that he was one of the least-wealthy members of the U.S. Senate during his 36 years there.

Biden argues he is uniquely qualified to win over voters in working-class communities that have abandoned a Democratic Party they see as elitist and out of touch. Biden’s appeal to such voters has played a big role in elevating him to front-runner in polling for the race.

But since leaving the White House, Biden’s lifestyle has hardly been middle-class. After signing a multimillion-dollar, three-book deal with Flatiron Books, Biden and his wife, Jill, purchased a six-bedroom vacation house in Rehoboth Beach, Del., for $2.7 million. The suburban Washington mansion he lives in is a sprawling home with a facade resembling the White House.

Biden’s swelling bank account — which is still only a tiny fraction the size of Trump’s purported wealth — may ultimately be a nonissue in the campaign. He’s not the only Democrat in the field with big figures in the bank. Even self-identified socialist Bernie Sanders of Vermont saw his income soar past $1 million recently, thanks to book royalties. Sens. Kamala Harris of California and Elizabeth Warren of Massachusetts are also millionaires. Tom Steyer, a billionaire, joined the race Tuesday morning.

The Biden campaign described the reports released Tuesday as a measure of his commitment to transparency.

“Today’s releases, in combination with the 10 years of taxes he released during the campaign in 2008 and his regular release as vice president, mean that Vice President Biden has now made public the last 21 years of tax returns — more than any other candidate currently running for president,” the campaign said in a statement.

Since he left office, one of Biden’s main institutional affiliations has been the University of Pennsylvania, where he was Benjamin Franklin Presidential Practices professor. From Jan. 1 of last year until May, Biden collected $540,484 in salary there. He has been on unpaid leave from the university since April.

Jill Biden also worked as a college professor, but even though she has made a career of teaching, she was paid a sliver of what her husband made — less than $100,000 a year. According to the American Assn. of University Professors, the average salary for full professors in 2017-18 was $104,820.

But the lion’s share of Biden’s new-found wealth came as a result of the sale of the former vice president’s 2017 book, “Promise Me, Dad,” and his speaking fees.

Candidates’ lucrative speaking fees have been a point of contention in recent years, especially for Democrats. Hillary Clinton faced repeated questions about paid speeches in her 2016 presidential campaign.

Clinton’s financial disclosure in 2015 showed she and her husband, former President Clinton, had banked more than $25 million since the start of 2014, collecting an average of $250,000 per appearance — including before major corporations like Goldman Sachs.

Sensitive to the complaints Clinton faced as she tried to keep some of her speeches private, the Biden campaign made a point of saying that more than half his speaking engagements were open to the media.

According to Biden’s financial reports, his speaking fees were as high as $234,820 in one case. When Biden gave a lecture at Drew University in New Jersey in March 2018, he was paid $190,000. A couple of weeks later, he was paid $180,000 to speak at Vanderbilt University in Nashville.

A Biden campaign official said the large speaking fees from universities came from special funds not linked to school tuition. Still, large fees from schools could be an issue at a time Biden and other candidates have made the rapid rise in the cost of attending college a major campaign issue.

Biden may have avoided some of the landmines that hurt Clinton. While the details provided on the financial disclosures are vague for some of his events, there do not appear to be any checks collected from Wall Street investment banks or other big corporate interests.

Overall, the documents showed the Bidens reported income of $396,456 in 2016, more than $11 million in 2017 and $4.6 million in 2018.

The couple paid federal income tax at a rate of 23.5% in 2016, 33.9% in 2017 and 33.4% in 2018.

Their charitable donations rose with their income: In 2016, the Bidens gave 1.5% of their income to charity; in 2017, they gave 9.2%; and in 2018, they donated 6%.

Sizeable chunks of their donations went to nonprofits named for the Bidens. Of the $1 million the Bidens donated to charity in 2017, $100,000 went to the Joseph Biden Foundation Community Legal Aid Society Inc., and $150,000 went to the Beau Biden Foundation for the Protection of Children, an organization named for Biden’s son, who died of brain cancer in 2015. An additional $55,000 went to Biden-related charities in 2018.

Also in 2018, the Bidens reported a $25,000 contribution to the International Assn. of Firefighters Foundation. The group found a way to thank him: When Biden launched his campaign earlier this year, the firefighters union was the first to endorse him.