Cohn became obsessed with publicizing his accomplishments — “I didn’t count it as winning if the world didn’t know about it” — and cultivated gossip columnists to stay in the headlines. (Not that he needed much help: Cohn’s role in the prosecution of Julius and Ethel Rosenberg established his name in legal and anti-Communist circles, even before his work for McCarthy made him a national figure.) To Cohn, “personal loyalty was everything,” Bird writes, and his disregard for legalities was captured in his maxim “Don’t tell me the law. Tell me the judge.”

He was even terrible at paying his bills, “another lesson Trump learned from Cohn,” Bird writes. Like his protégé, Cohn was indicted several times and was constantly under I.R.S. audit. At his birthday party in 1980, friends presented Cohn with a cake featuring his own scowling visage. (No word if T-shirts were available.) People who knew both men recognized that some of Trump’s verbal tics — “if you want to know the truth” and “I can tell you” — trace back to Cohn.

Cohn also liked to talk about crowd size. In 1963, after pleading not guilty to charges of perjury and obstruction of justice, Cohn stood outside a Manhattan courthouse and told the assembled reporters that it was the “biggest crowd I’ve seen since the Army-McCarthy hearings.” And when he gave a party in Washington celebrating Ronald Reagan’s 1980 election, Cohn boasted about the celebrities in attendance (Barbara Walters and Andy Warhol, among others) and bragged that “we had the biggest crowd.”

Oh, and Cohn sported a permanent tan, too.

Beyond style and temperament, Cohn and Trump shared a political template. Cohn “excelled at performative politics,” Bird writes, purporting to stand for the working class while riding around in a Rolls-Royce or hosting parties aboard his yacht, the Defiance. Both men saw history “through a lens tinted by conspiracy,” branding their rivals as Communists, radicals or denizens of the “deep state.” In a 1978 profile by Ken Auletta, which Bird cites, Cohn assails liberals for believing that “we must be loved by the rest of the world, which happens to hate us,” rather than focusing on national interests. “There’s nothing particularly original about Trump’s macho instincts in foreign policy,” Bird argues. They came from Cohn.

“The McCarthyism of the 1950s,” Bird writes, “morphed into a nativist, America Firster politics that transformed the modern Republican Party into a party of resentments against the old establishment, against multiculturalism, globalization and liberal culture” — with Cohn serving as connective tissue between McCarthy and Trump. Reading “American Scoundrel,” I sometimes wondered why the title wasn’t plural.