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Democrats were out in full force on Sunday blasting the idea of an independent presidential bid by longtime Democrat and billionaire businessman Howard Schultz, who said he was "seriously thinking" of doing so.

Julián Castro, a former Housing and Urban Development secretary under President Barack Obama and a contender for the 2020 Democratic presidential nomination, urged the former Starbucks CEO not to run.

"I have a concern that if he did run, that, essentially, it would provide [President] Donald Trump with his best hope of getting re-elected,” Castro said on CNN's "State of the Union."

Neera Tanden, president of the Center for American Progress, a liberal think tank, went further than Castro, tweeting that "vanity projects that help destroy democracy are disgusting."

"If he enters the race, I will start a Starbucks boycott because I’m not giving a penny that will end up in the election coffers of a guy who will help Trump win," she said.

In a "60 Minutes" interview that aired Sunday, Schultz, who is worth $3.3 billion according to Forbes, said he would seek the presidency as a "centrist independent."

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"We're living at a most-fragile time, not only the fact that this president is not qualified to be the president, but the fact that both parties are consistently not doing what's necessary on behalf of the American people and are engaged, every single day, in revenge politics," he told CBS's Scott Pelley.

Trump tweeted Monday morning that Schultz didn't have the "guts" to challenge him for the presidency.

Howard Schultz doesn’t have the “guts” to run for President! Watched him on @60Minutes last night and I agree with him that he is not the “smartest person.” Besides, America already has that! I only hope that Starbucks is still paying me their rent in Trump Tower! — Donald J. Trump (@realDonaldTrump) January 28, 2019

Schultz said he would run as an independent instead of as a Democrat because "we see extremes on both sides." Asked if he was concerned about siphoning votes away from Democrats and assisting Trump in getting re-elected, Schultz said he wants to "see the American people win."

"I don't care if you're a Democrat, Independent, Libertarian, Republican," he said. "Bring me your ideas. And I will be an independent person, who will embrace those ideas. Because I am not, in any way, in bed with a party."

Schultz bashed Medicare-for-all, a major policy being embraced by Democrats at the moment. Schultz said the proposal is "as false as the wall."

In an interview with The New York Times, Schultz said he planned to tour the country for the next three months promoting his new book before deciding whether to ultimately run.

Though independent and third-party candidates have affected past presidential races, none have won the presidency in modern times.

Former New York City mayor and fellow billionaire businessman Michael Bloomberg, who is considering a Democratic presidential bid of his own and has flirted with independent runs in the past, offered strong criticism on Monday.

"Given the strong pull of partisanship and the realities of the electoral college system, there is no way an independent can win," Bloomberg said in a statement. "That is truer today than ever before."

"In 2020, the great likelihood is that an independent would just split the anti-Trump vote and end up re-electing the President," he continued. "That's a risk I refused to run in 2016 and we can't afford to run it now."

Tina Podlodowski, chair of the Washington State Democratic Party, said in a Monday statement that Schultz's run "isn't about bringing anyone together" and was "about one person: Howard Schultz."

"An independent bid for president has not worked in the past, and it won’t work this time," she said of the executive who made his career in The Evergreen State. "Howard, if you want to run for President, run as a Democrat. If you want to rely on your money to get elected instead of talking with voters, run as a Republican."

The state party pushed back on the idea of an independent Schultz bid in a Sunday tweet featuring a Starbucks cup.

Not all Democrats agreed on the threat Schultz posed. Democratic Rep. Eric Swalwell of California, himself a possible 2020 candidate, tweeted that he "could care less" if Schultz sought the presidency via an independent bid.

"If we do this right, our Democratic ideas and nominee will unite most Americans and replace @realDonaldTrump," he tweeted. "Let’s focus on doing that."