Sen. Elizabeth Warren (D-Mass.) tore into former Starbucks CEO Howard Schultz on Tuesday after he mocked her plan to raise taxes on the very wealthy as nothing more than a “ridiculous” publicity stunt.

Schultz, who’s thinking about running for president as an independent in 2020, slammed Warren’s proposal to impose a 2 percent wealth tax on net worth above $50 million and a 3 percent tax on net worth above $1 billion during an interview on NPR’s “Morning Edition.”

“When I see Elizabeth Warren come out with a ridiculous plan of taxing wealthy people a surtax of 2 percent because it makes a good headline or sends out a tweet, when she knows for a fact that’s not something that’s ever gonna be passed ― this is what’s wrong,” Schultz said. “You can’t just attack these things in a punitive way by punishing people.”

Warren, who launched an exploratory committee for a 2020 presidential bid last month, fired back at the billionaire coffee kingpin later that morning.

“What’s ‘ridiculous’ is billionaires who think they can buy the presidency to keep the system rigged for themselves while opportunity slips away for everyone else,” Warren wrote on Twitter. “The top 0.1%, who’d pay my #UltraMillionaireTax, own about the same wealth as 90% of America. It’s time for change.”

What's “ridiculous” is billionaires who think they can buy the presidency to keep the system rigged for themselves while opportunity slips away for everyone else. The top 0.1%, who'd pay my #UltraMillionaireTax, own about the same wealth as 90% of America. It's time for change. https://t.co/D04G5fNvpa — Elizabeth Warren (@ewarren) January 29, 2019

Later on Tuesday, Warren threw more cold water on Schultz’s political aspirations.

“We have a billionaire who says he wants to jump into the race and the first issue he’s raised is no new taxes on billionaires?” she said to HuffPost. “Let’s see how that goes.”

Schultz announced that he is “seriously considering” a presidential run on Sunday, drawing outrage from Democrats who worry an independent candidate would help President Donald Trump win a second term. A viable independent candidate could attract Democrats and Republicans alienated by Trump’s presidency, potentially fracturing the electorate in ways that boost him in 2020.

Billionaire Michael Bloomberg, who himself is flirting with a 2020 run, warned on Monday that a third-party candidate would have no chance of winning the election.

“I have seen enough data over many years to know that anyone running for POTUS as an independent will split the anti-incumbent, anti-Trump vote,” Bloomberg wrote in a statement. “The stakes couldn’t be higher. We can not afford the risk of spoiler politics that result in Trump’s re-election.”

Igor Bobic contributed reporting.

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This article originally appeared on HuffPost.