Former New York City Mayor Michael Bloomberg would pose the toughest challenge to President Trump’s re-election bid in 2020, a former Trump campaign manager said Wednesday.

Corey Lewandowski said he believes that Bloomberg, a billionaire media mogul, would be a serious threat to win the White House if he ran as a Democrat because of his vast fortune, name recognition and reputation as a job creator.

“Mike Bloomberg created tens of thousands of jobs over the course of his lifetime,” Lewandowski told reporters at a Christian Science Monitor breakfast in Washington, the New York Post reported. “That is a story the American people like -- and he’s an outsider. I think it would be a very competitive race.”

Lewandowski, now an adviser to Vice President Mike Pence's Great America PAC, said the self-made billionaire was "an American success story” whose ability to self-fund would benefit his campaign. Bloomberg's net worth is $52 billion, according to Forbes.

He said another advantage was Bloomberg's “history of running the nation’s largest city.”

Bloomberg, 76, is founder and CEO of Bloomberg LP, a global media company. The three-term New York City mayor initially ran and served as a Republican before becoming an independent after his second term. He was a Democrat prior to running for office.

“Mike Bloomberg created tens of thousands of jobs over the course of his lifetime. That is a story the American people like -- and he’s an outsider." — Corey Lewandowski, former Trump campaign manager

Lewandowski also said Bloomberg held enough clout on progressive issues to win presidential primaries and make a close general election, according to the Hill.

Bloomberg, an outspoken critic of Trump, has said he plans to spend at least $80 million on this year's midterm elections to help the Democratic Party gain a majority in the U.S. House.

He twice flirted with running for president as an independent candidate, but ruled it out in 2016, believing he wouldn’t win on a third-party ticket, the New York Post reported.

Other rumored contenders for the Democratic Party's 2020 presidential nomination include U.S. Sens. Elizabeth Warren, D-Mass., and Bernie Sanders, I-Vt., and former Vice President Joe Biden.

The Associated Press contributed to this report.