Former Starbucks CEO Howard Schultz said he is preparing to launch an independent campaign for president of the USA, rebuffing growing Democratic concern that such a move could help re-elect President Donald Trump.

"I am seriously thinking of running for president," Schultz, a self-described Democrat, said on CBS' 60 Minutes. "I will run as a centrist independent outside the two-party system."

A self-funded independent campaign by a well-known billionaire focused on attacking the two-party duopoly that has long defined presidential politics has the potential to reshape the dynamics of the race.

"We're living at a most-fragile time," Schultz said. "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."

Democrats fear a credible third-party candidacy could allow Trump to win states he otherwise would have lost or push a decision on the election to the US House, where Republicans have an advantage in the number of state delegations they control. The majority view of the 50 delegations, not actual control of the House, would determine the outcome of any such vote.

"If this is about principles and not ego, Schultz would help Democrats defeat Trump or run as a Democrat and broaden the debate within the party," said Center for American Progress President Neera Tanden, a former adviser to Hillary Clinton and Barack Obama in their 2008 campaigns. "Every value he claims to hold dear is undermined by a Trump re-election."

Former Housing Secretary Julian Castro, who is running for president in 2020 as a Democrat, said that he was also concerned, even as he praised Schultz for his business accomplishments.

As Castro and others suggested, their worry is that a three-way race would split Trump's opposition, while the president's own support would remain intact.

"I have a concern that, if he did run, that, essentially, it would provide Donald Trump with his best hope of getting re-elected," Castro said on CNN's State of the Union. "I would suggest to Mr. Schultz to truly think about the negative impact that might make."

The potential run also risks ensnaring Schultz's company, where he remains chairman emeritus. The Washington state Democratic Party tweeted a photo Saturday of a Starbucks coffee cup with the words "Don't do it Howard!" written on the side.

The last time a third-party presidential candidate won individual states was in 1968, when former Alabama Gov. George Wallace picked up 46 Electoral College votes in the South on the American Independent Party ticket. Republican Richard Nixon nonetheless won that election, and third-party bids have been blamed by partisans since then for affecting election results indirectly.

Ralph Nader's Green Party candidacy in 2000 attracted far more votes in Florida than the margin by which Republican George W. Bush won the decisive state over Democrat Al Gore. Some Republicans have blamed businessman Ross Perot's 1992 Reform Party bid for drawing votes away from President George H.W. Bush. Democrat Bill Clinton won that election with just 43 per cent of the popular vote.

Under the US Constitution, if no single candidate receives a majority of the Electoral College votes, which are awarded on a state-by-state basis, the election is decided by the House, with each state delegation receiving a single vote. In the current Congress, Republicans have more representatives in 26 of the 50 state delegations.

Former New York Mayor Michael Bloomberg, a former Republican who is now considering running for president as a Democrat, decided against an independent candidacy in 2016, after concluding that he could not win a majority of electoral votes in a three-way contest and might inadvertently help Trump's election. A person close to Schultz, speaking on the condition of anonymity to discuss internal deliberations, said Schultz would only run if he sees a path to victory in a three-way battle.

Schultz, 65, who Forbes has said is worth about $3.4 billion, appeared on60 Minutes at the start of a national book tour to share his vision for reimagining "the promise of America."

His interest in an independent campaign is premised on the recent ideological drift of both political parties, which have embraced more ideologically ambitious proposals in recent years, even as the number of Americans registering as independents has risen. Yet he also has said the country needs to "go after entitlements," programs like Social Security and Medicare, which most voters have long supported.

In the interview, he said he favoured a path to citizenship for undocumented immigrants now in the country, favoured rejoining the Paris climate accords and criticised the Democratic promise of "free health care" for all as unrealistic.