WASHINGTON (REUTERS) - Billionaire environmentalist Tom Steyer, a fierce critic of President Donald Trump who had pushed early for his impeachment, abandoned his bid for the 2020 Democratic presidential nomination on Saturday (Feb 29) after trailing in third place in the South Carolina primary, a campaign source told Reuters.

Mr Steyer dropped out of the race on the day of his strongest showing yet in a 2020 Democratic nominating contest, but he was far behind winner Joe Biden and second-place finisher Bernie Sanders.

"Honestly, I can't see a path where I can win the presidency," Mr Steyer told supporters in South Carolina.

"I love you very much, this has been a great experience, I have zero regrets. Meeting you and the people of America has been a highlight of my life."

The 62-year-old former hedge fund manager from San Francisco portrayed himself as a political outsider and blasted corporate money in US politics in July, when he joined a field of two dozen Democrats seeking to deny Mr Trump, a Republican, a second term.

He poured US$64.7 million (S$90 million) of his own wealth in January into his bid for the Democratic nomination, bringing his total campaign spending to $267 million.

Mr Steyer amassed a fortune, estimated by Forbes magazine at US$1.6 billion, after founding investment firm Farallon Capital Management in the mid-1980s and serving as a partner at San Francisco private equity firm Hellman & Friedman.

In January 2019, he had said he was passing on a 2020 run to focus on efforts to impeach Mr Trump and get Democrats elected to the US Congress.

Mr Steyer has been a force in Democratic fund raising over the past decade. During the 2018 election cycle, he was the second-largest donor to Democratic and liberal candidates and causes, according to the Centre for Responsive Politics.