Unthinkable is the perfect title for this extraordinary book, because it describes a superhuman feat.

Jamie Raskin is a fine writer, a Democratic congressman, a constitutional scholar and a deeply loving father. When 2020 began, he had no inkling that just 12 months later his country and his family would face “two impossible traumas”.

On 31 December, his beautiful, brilliant, charismatic 25-year-old son, Tommy, took his own life. Six days later, a vicious mob invaded Raskin’s workplace, the cradle of democracy, leaving several dead and injuring 140 police officers.

Raskin suffered “a violent and comprehensive shock to the foundations”. Never had he felt “so equidistant … between the increasingly unrecognizable place called life and the suddenly intimate and expanding jurisdiction called death”.

This is where the superhuman part came in. Instead of succumbing to unfathomable grief over the death of his son, Raskin seized a lifeline thrown by the House speaker, Nancy Pelosi, and agreed to lead the effort to impeach Donald Trump for inciting the riot which might have derailed the peaceful transition of power.

He found “salvation and sustenance … a pathway back to the land of the living”.

“I’m not going to lose my son at the end of 2020 and lose my country and my republic in 2021,” he told CNN, less than three weeks after Tommy’s death.

Raskin’s astonishing story of tragedy and redemption, of “despair and survival”, depended entirely on all the “good and compassionate people” like Tommy, “the non-narcissists, the feisty, life-size human beings who hate bullying and fascism naturally – people just the right size for a democracy … where we are all created equal”.

Tommy Raskin was the fourth generation in a great liberal family. His maternal great grandfather was the first Jew elected to the Minnesota legislature. His father, Marcus Raskin, was one of the earliest opponents of the Vietnam war when he worked in the Kennedy White House. In 1968, Marcus Raskin was indicted with William Sloane Coffin, Dr Benjamin Spock and others for conspiracy to aid resistance to the draft. When Raskin was the only one acquitted, he famously demanded a retrial.

Jamie Raskin taught constitutional law then ran for the Maryland Senate, with Tommy, then 10, his first campaign aide. In the state legislature, Raskin helped outlaw the death penalty and legalize same-sex marriage.

Tommy was a second-year student at Harvard Law School when Covid began. Like so many others with clinical depression, the catastrophe deepened his symptoms. His father described his illness as “a kind of relentless torture in the brain … Despite very fine doctors and a loving family … the pain became overwhelming and unyielding and unbearable at last.”

This is also a political memoir, of the Capitol attack and the second impeachment. Driving to the Capitol, Raskin spotted MAGA supporters heckling a young Black driver and a car with a bumper sticker reading: “If Guns Are Outlawed, How Am I Going To Shoot Liberals?”

He realizes these “fascist bread crumbs throughout the city” should have activated “some kind of cultural alarm”. More chillingly, he reports the decision of some Democrats to cross their chamber after Congress was invaded, “because they thought a mass shooter who entered would be less likely to aim at the Republican side of the House”.

But Raskin was never afraid: “The very worst thing that could ever have happened to us has already happened … and Tommy is with me somehow every step of the way. He is occupying my heart … He is showing me the way to some kind of safety … My wound has now become my shield of defense and my path to escape, and all I can think of is my son propelling me forward to fight.”

The most powerful part of Raskin’s book, the heart-shattering part, is his love letter to Tommy, a “dazzling, precious, brilliant … moral visionary, a slam poet, an intellectual giant slayer, the king of Boggle, a natural-born comedian, a friend to all human beings but tyrants and bullies, a freedom fighter, a political essayist, a playwright, a jazz pianist, and a handsome, radical visitor from a distant future where war, mass hunger and the eating of animals are considered barbaric intolerable and absurd”.

Raskin realized that for the last week of his life, his son had made an effort to impersonate someone in perfect mental health, so no one would intervene. These were his parting words: “Please forgive me. My illness won today. Please look after each other, the animals and the global poor for me. All my love, Tommy.”

Raskin takes some solace remembering the story of Abraham Lincoln’s son Willie, who died of typhoid fever at the age of 12 in 1862, plunging his parents into depression.

It had been a point of pride that Raskin responded to every constituent, but a deluge of condolences made that impossible. There was also a call from Joe Biden, three days after Tommy died. The president-elect promised “the day would come when Tommy’s name would bring a smile to my lip before tears to my eyes”.

Eventually Raskin was convinced to write one letter for everyone sending condolences, one for everyone who wrote about impeachment and a third for everyone who offered condolences and political solidarity. One actually wrote: “I was looking for a condolence card for the loss of your son which also said ‘and thanks for saving our country too’, but Hallmark apparently doesn’t make those.”

Naturally, one of Raskin’s son’s heroes was Wittgenstein, who believed the truth of ethical propositions is determined by the courage with which you act to make them real.

“On this standard,” Raskin writes, “there have never been truer ethical claims than the ones made by Tommy Raskin, because he was all courage and engagement with his moral convictions.”

May this book and Tommy’s example inspire us all to rescue our gravely beleaguered democracy.

• Unthinkable: Trauma, Truth, and the Trials of American Democracy is published in the US by Harper