In addition to Mr. Bannon, Mr. Trump is relying on the advice of Roger Ailes, the founder and recently ousted chairman of Fox News, and Roger J. Stone Jr., whose 2015 book, “The Clintons’ War on Women,” accused Mrs. Clinton of being a lesbian, shaming her husband’s sexual accusers and playing a role in the death of Vincent W. Foster Jr., the deputy White House counsel who committed suicide in 1993.

“This is beyond anything anyone even thought about in their worst nightmare,” Mickey Kantor, a longtime Clinton family friend who was the United States trade representative and secretary of commerce under President Bill Clinton, said of the new conservative coterie around Mr. Trump.

David Brock, an ardent defender of Mrs. Clinton, said, “The conspirators of the ’90s were a ragtag bunch operating in secrecy and as a fringe subset of the conservative movement.”

Mr. Brock, a charter member of that “ragtag bunch” before having a political conversion, added, “Today’s equivalent is more a conglomerate than a conspiracy.”

Indeed, Mr. Bannon’s website — which had over 18.3 million unique visitors in July, according to data from comScore — has featured stories about the “radical-feminist code” in Mrs. Clinton’s language; suggested that Khizr Khan, who criticized Mr. Trump at the Democratic National Convention and whose son, an Army captain, was killed in Iraq, should blame Mrs. Clinton for his son’s death; and said Mrs. Clinton’s team was straining to highlight “whatever scraps of humanity can be found in her soulless, ambition-laden heart.”

And as Mr. Trump has raised questions about Mrs. Clinton’s health, saying she “lacks the mental and physical stamina” to fight terrorism, Breitbart has amplified those claims. “Bizarre behavior, seizure allegations raise doubts about Hillary Clinton’s health,” read one headline last week. “Clinton crash,” began another, with a video of her briefly stumbling while on stage with Vice President Joseph R. Biden Jr.