At a rally in Omaha Wednesday night, billionaire investor Warren Buffett officially threw his backing behind the frontrunner in the Democratic field of presidential candidates. Buffett has said before that he would vote for Hillary and that he donated to her campaign. Buffett, the chief executive of Berkshire Hathaway (BRK-A), has written checks for candidates before. But it’s unusual for him to stump for a candidate.

Buffett and Clinton both spoke at the event about income inequality and their desire to raise taxes on the wealthy. Buffett praised Clinton, saying he believed the candidate “will make sure that those people who are having to work two jobs to barely get by will not have that kind of world for themselves and their children moving forward,” The New York Times reported. He recounted statistics showing that while the wealthiest Americans saw their incomes increase sevenfold in a two-decade span, their tax rate fell by one-third.

At the same event, Clinton said that she wanted to go further than President Obama in raising taxes on the wealthy. President Obama made the “Buffett Rule” part of his fight against inequality, proposing that no millionaire pay less than 30% in taxes on his or her income. (In 2011, Buffett famously wrote that he pays lower taxes than anyone in his office.) “The Buffett Rule says that millionaires should pay at least 30% income-tax rates…and I want to go even further,” Clinton said at the event, according to the Wall Street Journal.

Clinton gave no further details of her tax plan, but a campaign aide told the Journal that a plan will be unveiled in the coming year.

Buffett held fundraisers for both Clinton and Obama when they were competing for the Democratic nomination in 2008, The Wall Street Journal reported at the time, but did not officially endorse a candidate.