Billionaire Democratic donor Tom Steyer on Thursday blasted Howard Schultz as the former Starbucks CEO mulls an independent presidential bid, arguing Schultz "isn't ready for prime time."

Steyer, who earlier this month ruled out launching his own White House bid in 2020, chided Schultz over his criticism of Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE's (D-Mass.) "wealth tax" proposal. Schultz this week called the proposal a "ridiculous plan."

".@HowardSchultz isn't ready for prime time. His knee-jerk rejection of a wealth tax shows why. We can't end inequality until people like Howard & me are required to invest our fair share into our country. It's not 'punitive'—it's patriotic," Steyer tweeted.

.@HowardSchultz isn't ready for prime time. His knee-jerk rejection of a wealth tax shows why. We can't end inequality until people like Howard & me are required to invest our fair share into our country. It's not "punitive"—it's patriotic. — Tom Steyer (@TomSteyer) January 31, 2019

Warren, who has announced an exploratory committee for president, earlier this month proposed a 2 percent annual tax on individuals with a net worth above $50 million. Schultz blasted the plan, arguing the senator was pushing a "punitive" tax.

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"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 during an interview on NPR’s “Morning Edition."

“You can’t just attack these things in a punitive way by punishing people," he added.

Schultz has said he is considering a bid for the presidency as a "centrist independent," though his rollout this week has prompted fury on the left, with Democrats arguing his entrance into the race could divide the vote and help reelect President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE.

An adviser to Schultz maintained during an interview on CNN this week that the former coffee chain executive was intent on seeing Trump defeated.

"There's nothing more important to [Schultz] than beating Donald Trump. He thinks he’s fundamentally unfit for office," the adviser said.