The Clintons have no official role in Mr. Brock’s empire, but, along the way, as he has abandoned his right-wing roots and sought to expose what he views as the conservative machinery he was once a part of, they have encouraged his efforts.

On Monday night, before his speech, Mr. Brock was back at the Capital Hotel on West Markham Street in Little Rock. The last time he was at the wood-paneled bar, Mr. Brock, 51, said he was “still in my 20s, drinking” and “plotting a campaign of dirty tricks” to stop the Clintons. This time, he socialized with Skip Rutherford and Bruce Lindsey, two of Mr. Clinton’s oldest friends and aides.

“In the 1990s, this guy was as rough and tumble as they get,” Mr. Rutherford said in an interview. “I don’t know if this is a walk through nostalgia or waking up from a bad dream, but today David Brock reintroduces himself to Arkansas.”

In 1996, Mr. Brock wrote a flattering biography of the first lady and, later, publicly changed his political affiliation. His 2002 book, “Blinded by the Right: The Conscience of an Ex-Conservative,” provided a firsthand account of his evolution and resonated with the former president. Mr. Clinton was known to hand out copies of Mr. Brock’s book as proof that the “vast right-wing conspiracy” (a term Mrs. Clinton used in a 1998 television interview) existed.

The “super PAC” Mr. Brock founded, American Bridge, has tapped into a rich network of Clinton supporters. Among them, according to federal disclosures, are George Soros; Steve Bing; Stephen M. Silberstein, a Bay Area entrepreneur; and Susie Tompkins Buell, a friend of Mrs. Clinton’s based in San Francisco.

Last year, Mr. Clinton delivered the keynote address at a fund-raiser in New York for Mr. Brock’s biggest donors. Mr. Brock thanked the former president and Mrs. Clinton for “giving me the gift of forgiveness,” said one person who attended the fund-raiser, which was closed to the news media, but could not discuss the event for attribution.

These days, Mr. Brock has a gray pompadour, and often dresses entirely in black, making him a conspicuous presence in the Brooks Brothers world of political operatives here. He splits his time between Washington and the West Village in Manhattan, and he works in an open office with Toby, his schnoodle (a poodle-schnauzer mix), at his side.