Less than three months before the most consequential American election of modern times, Rick Perlstein has completed his epic history of the forces that ultimately put Donald Trump in power. Reaganland, Perlstein’s fourth volume on the rise of US conservatism from 1960 to 1980, is out on 18 August.

Including notes, the book runs to more than a thousand pages. An hour or so before we speak, however, Perlstein puts out a rather shorter statement: a tweet.

“Open letter to press. I’ve given my last interview about the ’68 election’s lessons for 2020. Given Trump’s tweet on postponing, the political media’s determination to bound its discussion within the frame of normal politics is downright dangerous, and I won’t be complicit.”

So I don’t ask Perlstein to compare Richard Nixon – the subject of volume two, Nixonland (2008) – with a president repeating his “law and order” message and who the same day says “universal mail-in voting” will lead to “the most INACCURATE & FRAUDULENT election in history”, and therefore suggests the US might “Delay the Election until people can properly, securely and safely vote???”

Trump can’t delay the election. But he can fight and lie and cheat, as Nixon did, and that seems likely to keep reporters knocking on Perlstein’s door. His tweet, he says, came “from a frustrating interview in which I said over and over again: ‘The real thing you need to be talking about as a reporter … is parallels to what was going on in South America in the 1970s and Europe in the 1930s.’”

His point is that when Trump “keeps doing things proven unpopular to all but the fascistically inclined, maybe he sees his audience as the fascistically inclined – those more useful to him for keeping permanent power than mere voters”.

On the phone, from Chicago, he expands.

“By talking about how many electoral votes [Trump’s] trying to get in the suburbs with his law-and-order appeal, you’re kind of doing active harm to contemporary understanding. That sort of consensus frame, that things aren’t really as bad as they seem, is a story that gatekeeping media elites tell about the world, that makes them actors in the story, not merely commentators.”

A man of the left, Perlstein agrees his books are as much about the failures of liberalism and the media as the success of the right.

“The only reason we’re talking about 1968 is because Trump wants to talk about 1968. He sets the agenda by tweeting ‘law and order’ and ‘silent majority’. That’s his story.

“My shorthand is history is process, not parallels. There really can’t be a historical parallel. You can’t step in the same river twice or even once because the thing that happened in 1968 happened and we were responding to what happened. Even if there are similarities."