Democratic presidential candidate Hillary Clinton smiles as Berkshire Hathaway Chairman and CEO Warren Buffett introduces her at a rally in Omaha on Aug. 1. (Andrew Harnik/AP)

Billionaire Warren Buffett has spent a lifetime building a financial empire, spawning reams of business advice books and folksy investing mantras. In the twilight of his career, he is also embracing a new role: political street-fighter. 

At a rally in Omaha on Monday, the business icon praised Hillary Clinton for her policies and condemned Donald Trump for refusing to release his tax returns. Buffett invoked Trump's record of bankruptcies and the failure of his only public company, Trump Hotels and Casino Resorts, which listed on the New York Stock Exchange in 1995. “In 1995, when he offered this company, if a monkey had thrown a dart at the stock page, the monkey would have made on average 150 percent,” Buffet said. “But the people that believed in him, that listened to his siren song, came away losing well over 90 cents on the dollar.”

Attending a campaign rally with Democratic presidential nominee Hillary Clinton in Omaha, investor Warren Buffett challenges Republican presidential nominee Donald Trump to exchange tax returns and answer questions. (Reuters)

Buffett, who turns 86 later this month, has long-supported left-leaning causes. But over the past decade-and-a-half, he has fought more loudly for Democrats against Republican proposals. His voice has been especially resonant given that corporate America has long been more friendly with Republicans. But the Oracle of Omaha has thrown money behind a number of liberal candidates and causes, from the campaigns of John Kerry, Barack Obama and Hillary Clinton to issues like abortion rights and environmental protection.

Buffet's most high-profile impact on policy may have been lending his name to an Obama administration proposal to raise the minimum tax rate so that people who make more than $1 million would pay at least 30 percent of their income in taxes, which became known as “the Buffett rule." Buffett had famously commented that he paid a lower tax rate than his secretary, due to loopholes that allow income from investments to be taxed at a much lower rate than other income.

Gene Sperling, who advised Obama at the time as the National Economic Council director and currently advises Clinton's campaign, had given the billionaire a call to see if he would allow the administration to brand the proposal with his name. "If all the diseases are taken, I'll take a tax rule," Buffett quipped.

“He knew he’d be putting himself in the firing line by letting his name be used for that," Sperling recalled. The use of Buffett's name increased the prominence of the proposal, but it also intensified the personal attacks and scrutiny that Buffett received, Sperling said. "So it was not a tiny thing."

Trump reacted strongly on Tuesday to Buffett's remarks in Omaha, telling Fox Business Network, "I don’t care much about Warren Buffett." He added, "He's been a Hillary fan for a long time and I think that's fine. Hillary is a disaster."

Buffett's support for left-leaning causes hasn't always been unbridled. He has been a strong advocate for fiscal discipline, for example cautioning against overspending on Social Security. He has also largely refrained from supporting left-leaning super PACs, whose influenced he opposed in principle. (There's a glaring exception: He donated to a pro-Clinton super PAC in 2014, later saying he didn't realize it was a super PAC.)

Buffett actually started his adult life as a Republican, largely due to the influence of an arch-conservative father, a stockbroker and four-time Republican congressman who distrusted Franklin Delano Roosevelt’s New Deal and believed that the U.S. should maintain the gold standard and stay out of World War II.

Buffett greatly admired his father, but “grew up in a different world,” said Roger Lowenstein, the author of the biography “Buffett: The Making of an American Capitalist.” Born just 10 months after the stock market crash of 1929 and growing up amid the Great Depression, Buffett saw how public investment and welfare programs helped revive the U.S. economy.

His wife, Susie, whom he married in 1952, swayed him somewhat to the left. But Buffett's break with the Republican Party really came during the Civil Rights era, as he saw the party largely remain on the sidelines in a fight over social injustices, Lowenstein says.

In 1965, Buffett bought a run-down textile mill called Berkshire Hathaway. He rebuilt it, and over the following decades used the profits to create a multibillion-dollar holding company with investments in the insurance industry and some of the country's best-known consumer brands, including Coca-Cola, Dairy Queen and Fruit of the Loom. In the process, he amassed a personal fortune of more than $60 billion, ranking him as the world’s third-richest man in 2016.

Buffett forged a close relationship with former Washington Post publisher Katherine Graham — he was one of the paper's biggest investors — which brought him into contact with Washington's political set during the 1980s and 1990s. Buffett's biggest initial foray into national politics came in the early 2000s when he joined George Soros and Bill Gates Sr., the father of the world's richest man, in criticizing President George W. Bush's proposal to cut taxes by $1.6 trillion over the next 10 years.

Bush had proposed doing away with the estate taxes levied on the net worth of an individual when they die, a change Buffett said would result in an enduring aristocracy of the wealthy. He criticized the measure as akin to ''choosing the 2020 Olympic team by picking the eldest sons of the gold-medal winners in the 2000 Olympics.''

He went on to serve as an economic adviser to Arnold Schwarzenegger's campaign for California governor in 2003 and John Kerry's presidential campaign in 2004. In 2006, Buffett once again made headlines by pledging 85 percent of his wealth — then more than $37 billion — to charity. The bulk went to the Bill & Melinda Gates Foundation, while a smaller sum went to foundations variously set up in the name of his late wife and run by his three children, which supported causes like antinuclear proliferation and education for low-income children.

Buffett has long been a supporter of Hillary Clinton, backing her Senate campaign in 2000 — and his previous statements suggest he likes the idea of a female president. Buffett has said that one reason for his success was that he was only competing with half the population.

But Buffett also appears to support Clinton's broader economic vision for the country. “He grew up in egalitarian Nebraska, where there were very few rich people, and with the exception of the Depression, most people were middle class," says Lowenstein. "He cares about retaining that quality in American society.”

Buffet's office did not respond to a request for comment for this article.

At the Omaha rally on Monday, Buffett strove to present his biography as a counterweight to Donald Trump’s success story. He drew parallels between his own life and Trump’s as he rebuked the candidate for criticizing the Muslim American parents of an Army officer who was killed in Iraq. “Donald Trump and I haven’t sacrificed anything,” he said, noting that no member of the Buffett or Trump family had gone to Iraq or Afghanistan.

He also challenged Trump to release his tax returns, something Trump had previously declined to do, citing an ongoing audit. Buffett, who said he is also being audited, promised to release his tax returns if Donald Trump would.

Buffett assured the gathering in Omaha that their votes mattered, especially due to Nebraska's quirky electoral college rules. Unlike most states, Nebraska allows its districts to award electoral college votes to different parties, meaning the more liberal city of Omaha has sometimes diverged from the sea of red that makes up the rest of Nebraska to award a vote to the Democratic Party.

Buffett committed to personally driving at least 10 people to the polls in November. The octogenarian businessman is still known for driving himself to work every morning.

"He is the only person of his stature who when he says, 'Can I get you a ride to the airport?' means can I get my car from the garage and personally drive you to the airport," said Sperling, laughing. "So you can imagine the difference between him and Donald Trump and their style."

See also:

The myth and the reality of Donald Trump’s business empire

The strange and wonderful things people searched for during the Republican and Democratic conventions

Wall Street is sitting on billions meant for American charities