Democratic presidential hopeful former Vice President Joe Biden speaks during the sixth Democratic primary debate of the 2020 presidential campaign season co-hosted by PBS NewsHour & Politico at Loyola Marymount University in Los Angeles, California on December 19, 2019.

A longtime fundraiser for President Barack Obama who recently helped former Texas Rep. Beto O'Rourke in his bid for president is opening his donor network to former Vice President Joe Biden.

Louis Susman, a former U.S. ambassador to the United Kingdom under Obama, told CNBC in an interview that he's going to start fundraising for Biden in the wake of O'Rourke dropping out of the race in November.

"I'm committed to defeating President Donald Trump, and I think he's [Biden] the most capable person to defeat Trump and to be president," Susman said late Friday. He said he does not know yet when he is going to host his first event for Biden.

Biden's pickup of Susman, who currently is a senior advisor to behemoth asset management firm Perella Weinberg Partners, is the latest boost to a campaign that's looking to start 2020 off with a clear advantage in the fundraising game. Wall Street executives Marc Lasry and Blair Effron recently signaled to allies that they, too, are going to back Biden, after they previously supported Sen. Kamala Harris.

Susman's backing of Biden comes after he had a strong debate performance on Thursday.

Before becoming an ambassador, Susman bundled at least $500,000 during Obama's first run for president in 2008, according to the nonpartisan Center for Responsive Politics. He was also the national finance chairman for John Kerry's 2004 bid for president.

Biden is expected to have better fourth-quarter fundraising results than his third-quarter efforts, where he finished bringing in only $15 million and had only $9 million on hand.