Mr. Stone, 64, has a fashion blog and likes to quote Gore Vidal’s advice to “never miss a chance to have sex or appear on television.” He divides his time between Fort Lauderdale, Fla., and an apartment on the Upper East Side of Manhattan crammed with political memorabilia and Le Corbusier furniture.

Image Mr. Brock, 53, divides his time between Washington and the West Village in Manhattan, throwing lively salons and wooing liberal donors on both coasts, often accompanied by Toby, his schnoodle — a schnauzer-poodle mix. Credit... Danny Johnston/Associated Press

“Socially, he’s a very charming, likable, intelligent guy,” Mr. Stone said of his rival in a telephone interview on Saturday. He praised Mr. Brock’s style, saying he is “a dapper guy” and likening his hairstyle to that of the title character in “Eraserhead,” David Lynch’s 1977 surrealist horror film.

But in recent weeks, as sexual assault allegations against former President Bill Clinton surfaced in the campaign, the intersections of Mr. Brock’s past with Mr. Stone’s attacks on the Clintons have made for a deeper kind of intrigue.

Last week, when Mr. Trump brought up a decades-old rape allegation against Mr. Clinton in a Fox News interview, Mrs. Clinton’s allies saw the influence of Mr. Stone. His thinly sourced 2015 book, “The Clintons’ War on Women,” which he wrote with Robert Morrow, focuses on Mr. Clinton’s sexual misconduct and accuses Mrs. Clinton of silencing women who came forward to complain about it.

But it is reporting by Mr. Brock that Mr. Stone has used to help Mr. Trump make that case.

Before Mr. Brock became the man at the center of a multimillion-dollar operation built to defend the Clintons, he was a self-described conservative hit man intent on taking them down. He and Mr. Stone knew each other socially and would occasionally compare notes.

Reporting for The American Spectator, a conservative newsmagazine, Mr. Brock asserted in 1994 that Arkansas state troopers facilitated sexual liaisons for Mr. Clinton when he was the state’s governor, allegations that have been central to Mr. Stone’s attacks.

“Today, Brock claims his American Spectator stories exposing Bill Clinton were false,” Mr. Stone wrote in his book. “He’s lying.”