In the heat of the 2000 presidential campaign, Vice President Al Gore took a break from barnstorming battleground states to attend a fund-raiser for the Democratic National Committee in East Hampton, N.Y. Standing behind Mr. Gore onstage was Scott Bessent, a hedge fund manager and — at the time — a major donor to Democrats who co-hosted the event at his home.

On Friday evening, Mr. Bessent was tapped by President-elect Donald J. Trump to be his Treasury secretary. Having won the trust of Mr. Trump and his inner circle, Mr. Bessent would lead a Republican economic agenda of cutting taxes, culling federal regulations and enacting sweeping tariffs.

The selection caps an extraordinary career arc for an investor who was once a protégé of the liberal billionaire philanthropist George Soros and who gave money to top Democrats including Hillary Clinton, John Kerry and Barack Obama.

“He was very supportive of the causes and the people that we supported,” said Will Trinkle, a Democrat who co-hosted the event with Mr. Gore. He noted that Mr. Bessent, who would be the first openly gay Treasury secretary, was a strong advocate for gay rights and marriage equality.

If confirmed by the Senate, Mr. Bessent would help Mr. Trump as he attempts to remake the U.S. economy. As Treasury secretary, Mr. Bessent would work to steer tax cuts through Congress, lead trade negotiations with China and help cull federal regulations that Mr. Trump believes are stifling the economy.