Jeffrey Toobin has combined two great books in one. The first is an edge-of-your-seat thriller, describing Timothy McVeigh’s every movement on his way to committing one of the most horrific crimes in American history. The second traces how a huge part of the Republican establishment has come to embrace many of McVeigh’s most dangerous convictions.

Toobin is a lawyer who became a full-time writer and TV pundit 30 years ago. This is his ninth book and his most important, because it gives the clear and present danger of rightwing extremism the attention it deserves.

McVeigh was a brilliant marksman who fought in the first Iraq war but failed a tryout for the Green Berets after only two days. This, Toobin writes, was “a shattering defeat … he had no plan B”.

McVeigh’s biggest ideological influence was a novel, The Turner Diaries, which envisaged a world in which the government had the power to confiscate private arms, Black people were allowed to attack whites with impunity and whites were punished for defending themselves. It also imagined the blowing up of the FBI building in Washington with a truck filled with thousands of pounds of fertilizer, blasting gelatin and sticks of dynamite.

That became McVeigh’s template for blowing up the Alfred P Murrah Federal Building in Oklahoma City, on 19 April 1995, with a rental truck. The death toll was 168, including 19 children. More than 500 were injured.

Toobin covered McVeigh’s trial for the New Yorker and ABC News. His interest was rekindled when he realized the conspirators arrested in a plot to kidnap the current Michigan governor, Gretchen Whitmer, were much like McVeigh.

“I know these people,” he writes.

Then he discovered McVeigh’s lead lawyer had donated 635 boxes of documents to the University of Texas.

“I knew that an archive of this extent … had never before been publicly available in a major case.”

In Homegrown, Toobin combines the fruits of those documents with interviews with more than 100 participants, among them Bill Clinton, president at the time, and Merrick Garland, now attorney general, then lead prosecutor of McVeigh. The result is one of the most detailed and exciting true crime stories I have ever read.

But in many ways the other book Toobin has written is even more important. It is the book that looks at the birth of the extreme language that now dominates Republican politics. McVeigh embraced white supremacy and violent action just as a Republican House speaker, Newt Gingrich, and a talk show host, Rush Limbaugh, were engaging in “rhetorical violence at a pitch the country had rarely heard before on national broadcasts”.

Gingrich instructed Republicans to describe Democrats as sick, pathetic, traitors, radical and corrupt, while describing himself as standing “between us and Auschwitz”. Limbaugh said the “second violent American revolution” was a quarter of an inch away. Toobin draws a straight line to the titles of books written by extremists today, from Deliver Us From Evil: Defeating Terrorism, Despotism and Liberalism (Sean Hannity) to Treason: Liberal Treachery from the Cold War to the War on Terrorism (Ann Coulter).

To Toobin, the mistake Garland made was the same one he himself made when he covered McVeigh’s trial: both focused on the acts of a loner, instead of connecting the atrocity to the beginnings of the mainstreaming of rightwing extremism.

One of the most interesting parts of the book lies in Clinton’s prescience. Because the Oklahoma bombing came barely two years after the first bombing of the World Trade Center by Islamic fundamentalists, the media and many others assumed foreigners were the culprits again. Clinton was certain that wasn’t the case.

“This was domestic, homegrown, the militias,” he told his staff. “I know these people. I’ve been fighting them all my life.”

Clinton’s earliest political memory was of the Arkansas governor Orval Faubus refusing to allow Black students into Little Rock Central high school. Clinton remembered those opposed to integration, “the faces twisted with rage”. He also believed hatred was especially virulent in the early 1990s, because of “Gingrich’s sneering contempt and Limbaugh’s roiling bombast”.

McVeigh’s own linear connection to old hatreds was confirmed by his membership of the Ku Klux Klan.

McVeigh saw himself as the leader of an army of extremists but Toobin is convinced, by the evidence, there were only two significant co-conspirators. The disastrous change in our own time lies in the way the internet has enabled millions of such people to connect. One study for the Department of Homeland Security found social media was used in 90% of US extremist plots.

Toobin writes: “More than any other reason the internet accounts for the difference between McVeigh’s lonely crusade and the thousands who stormed the Capitol on January 6.”

The terrorism expert Juliette Kayyem said the internet gave “white-supremacist terrorism … what amounts to a dating app online”. In Toobin’s words, when Donald Trump became president, “the wolf pack had a new leader”.

This is one of the most important markers of the decline of the Republican party. “After Oklahoma City, no politicians defended … the attack.” But after January 6, many Republicans did just that. Andrew Clyde, a Georgia congressman, said the riot resembled “a normal tourist visit”. Since leaving the White House, Trump has “turned to a new level of feral zealotry”, embracing QAnon, the antisemitic conspiracy theory he reposts on his social media platform, and regularly expressing eagerness to pardon rioters as soon as he enters the White House again.

This week provided the most dramatic evidence yet of how completely the political-media establishment has been corrupted by organized hatred. CNN, a formerly respectable organization, decided the best thing it could do was to give a national forum to the hero of millions of white supremacists. That decision alone drove home the urgency of the message of Toobin’s brilliant book.

• Homegrown: Timothy McVeigh and the Rise of Right-Wing Extremism is published in the US by Simon & Schuster