Not so long ago, fake news stories were routinely smothered, simply by being ignored by the biggest newspapers and the major TV networks, their storylines safely confined to the National Enquirer and its tabloid competitors.

Rogue legislators with histories of racism or addiction to conspiracy theories usually suffered the same fate for the same reason – nobody gave them ink or air time. Their leaders in the House and Senate could complete their marginalization.

These gatekeepers did not have perfect judgement, but in our time it has become obvious that they provided essential protections for democracy. The internet and its infernal algorithms are the main reasons no institution or congressional leader retains the power to protect the public from outright insanity.

Robert Draper’s new book about Washington in the 18 months after January 6 is all about the fatal consequences of the brave new world the internet created, in which Republican outliers the like the Arizona congressman Paul Gosar and his mentee, Marjorie Taylor Greene of Georgia, are much more likely to be rewarded for “outrageous, fact-free behavior” than to be penalized for it.

The author, a New York Times magazine contributor, begins with a confession: all his previous books and articles about the Republican party “tended to bear the telltale influence of my father, a lifelong Republican”.

Since his focus is “the tension between the party’s reality-based wing and the lost-its mind wing”, this confession reinforces the idea that all the book’s harsh judgements are coming from a dispassionate observer.

But later on in the book this feels less like a confession and more like a mea culpa, when Draper describes three common notions about Donald Trump’s successful putsch: the idea it was accomplished through “force and surprise”; the notion “that the party was fully functioning and purposeful” before Trump took it over; and the contention “that the GOP bore no responsibility for the crime committed against it”.

As Draper writes, “Each of these notions is false.”

Unlike Mark Leibovich’s recent book, Thank You for Your Servitude, which covers much of the same territory but does not manage to tell us anything new, Draper provides pungent new anecdotes about and original analysis of the most outrageous actors, like Gosar and Greene, and their main enabler, the House minority leader, Kevin McCarthy.

Gosar spent a decade in Congress “building a portfolio of outrageous conduct even before social media’s ‘attention economy’ was fully capable of rewarding him for it”. One Gosar staffer was “advised by a top Republican operative, ‘You need to get out of there, that man is insane”. Another GOP aide called the congressman “my nominee to be that guy who comes in with a sawed-off shotgun one day”.

But what Draper finds most astonishing is that Greene, who attributed forest fires to (possibly-Jewish connected) space lasers and openly promoted QAnon conspiracies, would only need a year in Congress before becoming “the party’s loudest and most memorable messenger outside of Trump himself”.

Draper provides an excellent description of how Greene’s personal wealth and determination made it possible for her to move to an adjoining district and win the primary after the incumbent retired. She loaned her own campaign $500,000 and by March 2020 the extreme House Freedom Caucus had contributed nearly $200,000 more.

After she won the first round in her primary, before the run-off, Politico ran this pithy summary of her greatest hits: Greene “suggested that Muslims do not belong in government; thinks black people ‘are held slaves to the Democratic party’; called George Soros … a Nazi, and said she would feel ‘proud’ to see a Confederate monument if she were Black because it symbolizes progress made since the civil war”.

McCarthy and the rest of the House leadership denounced her. But then a funny thing happened – “or rather did not happen – back in Georgia. The attack on Greene by “fake news” and “the equally fake Republicans” delighted her new constituents and she won the run-off by 14 points. At her victory party, she said of Nancy Pelosi, the Democratic House speaker: “We’re going to kick that bitch out of Congress.”

The intellectual bankruptcy Draper chronicles pivots around McCarthy, whose blind ambition to become the next speaker leads to a series of despicable choices. First, he decides he must push Liz Cheney out of Republican leadership, because she refuses to pretend Trump lost the election because of fraud. Then he goes out of his way to mend his friendship with Trump and turn a blind eye to Greene’s outrages, because he is convinced he cannot win a House majority without Trump’s craziest supporters.

Draper makes a couple of small mistakes, describing an amendment McCarthy opposed that would have removed “language that could enable discrimination against LGBTQ+ members of the military”. The amendment actually would have banned military contractors from discriminating against LGBTQ+ employees, and it was debated five years after Congress finally ended discrimination against gay and lesbian sailors and soldiers. He also describes the New Jersey Democratic congressman Tom Malinowski as Jewish. He is not.

The exact moment the Republican party lost its soul probably came after the January 6 rioters tried to prevent the peaceful transfer of power to the duly elected new president by storming the Capitol – and a few hours later seven Republican senators and 138 representatives still voted to sustain spurious objections to the electoral votes of Pennsylvania.

McCarthy and Mitch McConnell, the Senate minority leader, were not among those election deniers – although McCarthy earlier voted to object to results from Arizona. But their refusal to convict Trump in his subsequent impeachment trial, or to stand up to any allies of the insurrection, guaranteed their party’s addiction to the lie that the presidential election was stolen.

Draper has performed an essential service by documenting the details of this singularly destructive cowardice.

• Weapons of Mass Delusion: When the Republican Party Lost Its Mind is published in the US by Penguin Press