Buffett, the chairman of Berkshire Hathaway -- commonly referred to as the oracle from Omaha -- joined Clinton at her first event in Nebraska and hosted a fundraiser for her campaign before the rally.
"I am going to fight hard to try and implement the Buffett rule," Clinton said before charging that Republicans want to "go back to the same failed economic policies again."
The Buffett rule, also known as the Fair Share Tax, is popular among Democrats, especially within the populist strain of the party who believe wealthy Americans get far too many benefits for the tax they pay. The tax would impose a 30% rate in federal taxes on anyone making more than a million dollars a year after charitable donations.
Clinton said that while she backs Buffett's rule, which was first pitched by the Obama White House in 2011, she wants to go further in ensuring wealthy Americans pay higher tax rates. Clinton did not detail those plans on Wednesday, but campaign aides told reporters that the candidate will be announcing details tax plans in the new year.
Clinton's tax plans for wealthy Americans and corporations are critical to her campaign because much of her proposed spending, on issues like debt free college, infrastructure spending and healthcare spending credits, are to be paid for by raising taxes on the wealthy.
Clinton, who has proposed $1 trillion in spending over 10 years, has also tied education tax credits, universal pre-kindergarten and a proposal to incentivize corporate profit sharing to raising taxes on the wealthy, closing corporate loopholes and other revenue enhancements.
"Everything that I am proposing I have a way to pay for it," Clinton said in Iowa earlier this year. "It's raising taxes on the wealthy, it's closing loopholes, it's doing what I believe we should do so that when I tell you 'here's my plan,' I will tell you how I want to pay for it. So I'm going to tell you what I will do and I will tell you how I will pay for it. And you should ask that of everybody."
President Barack Obama has included the rule in his 2016 budget proposal
, though Republicans have said no since it was first proposed five years ago.
Buffett endorsed Clinton's presidential run on Wednesday, telling the audience of around 800 people that the primary reason he is backing the former secretary of state is because of her plans to address economic inequality.
"What has happened is that despite this incredible bounty, millions and millions and millions of Americans have been left behind," Buffett said. "The game has been stacked in their direction. That is a primary reason... why I am going to be so delighted when Secretary Clinton takes the oath of office."
Clinton did not mention Tuesday night's Republican debate during her nearly hour long appearance, instead choosing to focus on policies she has already rolled out and political knocks against Republicans.
"I don't know why Republicans are playing to the lowest common denominator in their party," Clinton said.
After her Omaha rally, Clinton is scheduled to jump across the border to Iowa, where she will headline two town halls. The day of campaigning will be her last before the third Democratic debate on Saturday.