It is nearly half a century since Bob Woodward and Carl Bernstein became the world’s most famous journalistic double act, immortalized by Robert Redford and Dustin Hoffman in the film All the President’s Men.

But Woodward’s latest book, about the decline and fall of Donald Trump, is co-written with a different Washington Post colleague who was not yet born at the time of the Watergate break-in.

So, Robert Costa, aged 35, how does it feel to be the new Carl Bernstein?

“There is only one Carl Bernstein, to be sure, and I have immense respect for him,” Costa says by phone. “But we both had the privilege of working alongside Bob Woodward and when you work with Bob Woodward, you learn his method of reporting. He spends hours and hours interviewing people, probing them for better answers, candid answers about what really happened.”

Costa, who has spent much of his career as a daily political reporter at the Post, relished a chance to immerse himself in long-form investigative journalism. “Woodward would tell me, go back and keep digging, and I would have people over at my house and we would sit for hours with different sources.

“He would do the same and really talk to people in person. Get away from the phone, get away from email and do interviews in person. Bob Woodward is as old school as you can get and I think that’s a compliment in a digital journalism age where the frantic pace at times takes you away from the ability to really report at length.”

It is a characterization that resonates with the All the President’s Men depiction of Woodward and Bernstein diligently rifling through checkout cards at the Library of Congress or knocking on doors late at night and coaxing clues out of unwilling witnesses.

Woodward and Costa – time will tell if it has the same ring – interviewed Trump for the Washington Post in March 2016. The presidential candidate’s observation that real power derives from “fear” gave Woodward the title of his first book in what would prove a Trump trilogy: Fear (2018), Rage (2020) and, with Costa, Peril (2021).

The word haunts it from the opening page, which quotes Joe Biden’s inaugural address – “We have much to do in this winter of peril” – to the last sentence of the 418th and final page: “Peril remains.”

The authors interviewed more than 200 people (though both Biden and Trump declined) on “deep background”, resulting in more than 6,000 pages of transcripts covering the coronavirus pandemic, Black Lives Matter protests, presidential election, 6 January insurrection and nascent Biden presidency.

Among the more colorful anecdotes are Senator Lindsey Graham telling Trump: “You fucked your presidency up,” only for Trump to abruptly hang up on him, and Senator Mitch McConnell regaling Republican colleagues: “Do you know why [former secretary of state Rex] Tillerson was able to say he didn’t call the president a moron? Because he called him a ‘fucking moron’.”

Another piece of advice from Woodward, 78, to his co-author: assume nothing. Costa recalls: “I would have the two words ‘assume nothing’ taped to my computer screen because you can’t assume anything in American politics. We thought we knew the whole story of January 6 but realised it was not just a domestic political crisis. This was a national security emergency.

“This had the whole world on edge: the UK, the Russians, our European allies, the Chinese. President Trump’s conduct sparked a national security emergency that largely unfolded behind the scenes and that was the story we discovered and wanted to tell, along with the whole transition that our reporting shows was quite dangerous.”

This generated the splashiest news headlines from the book. The top US military officer, Gen Mark Milley, grew alarmed about Trump’s conduct and feared that he was in serious mental decline during his final days in office. At the same time Milley saw foreign allies and adversaries unsettled by the events of 6 January when a pro-Trump mob stormed the US Capitol and delayed ratification of Biden’s election win.

The chairman of the joint chiefs of staff decided to take action to avoid a potentially catastrophic miscommunication. Milley called Gen Li Zuocheng of the People’s Liberation Army in October 2020 and again in January 2021 to assure him that the US government was stable and had no intention of attacking China.

On the second call, two days after images of the deadly 6 January riot stunned the world, Milley sought to assure Li: “We are 100% steady. Everything’s fine. But democracy can be sloppy sometimes.”

And on that day Milley also told his staff that, if Trump ordered a nuclear strike, they would have to go through a consultation process with him before it was carried out – an effort to ensure that nothing be done in a rash manner.

After the book’s publication, Trump called Milley a “dumbass” and denied that he ever had any intention of attacking China. But that was not the point. Costa explains: “Milley, as a senior military officer, had many interactions with President Trump, and our reporting shows he does not believe President Trump sought war during the transition or even wanted to start a war.

“What Chairman Milley was worried about was a hair-trigger environment where the president’s conduct or different military exercises abroad could be misconstrued by the Chinese or others as some kind of rogue act that would lead to a confrontation that was unnecessary to the point of even being hostile and hot.

“He wanted to avoid that. De-escalation was the motivation for his efforts. Some people have read our book, including President Trump, and accused Milley of treason but our reporting doesn’t show any kind of intention to subvert the presidency. Our book shows him trying to preserve the American government from collapsing into some kind of military crisis or national security crisis.”

The implication that Milley worked around civilian leadership did indeed provoke uproar on the right and demands for his dismissal. But Milley’s office insisted that the calls were coordinated within the Pentagon and across the US government, while Biden said he had “great confidence” in the four-star general.

Peril also recounts an encounter in the Oval Office on the eve of the insurrection when Trump demanded that his vice-president, Mike Pence, overturn the results of the election, even though he lacked the constitutional power to do so.

Hearing the raucous cheers and blasting bullhorns of his supporters gathering in the streets of Washington, the president asked: “If these people say you had the power, wouldn’t you want to?”

Pence replied: “I wouldn’t want any one person to have that authority.”

Trump asked: “But wouldn’t it be almost cool to have that power?”

Pence said: “No … I’ve done everything I could and then some to find a way around this. It’s simply not possible.”

Trump shouted: “No, no, no!. You don’t understand, Mike. You can do this. I don’t want to be your friend anymore if you don’t do this.”

Torn between his oleaginous devotion to Trump and his oath to the constitution, Pence eventually turned to fellow Indianian Dan Quayle, who served as vice-president under George HW Bush. Quayle told him: “Mike, you have no flexibility on this. None. Zero. Forget it. Put it away.”

It is safe to assume that Woodward and Costa never imagined their book would contain a cameo by a man who once famously misspelled “potato” as the unlikely saviour of democracy. Costa says: “January 5th, 2021, will be remembered as the temptation of Mike Pence.

“You see him trying so hard to help President Trump, to do what Trump wants, but being told by his lawyers and his advisers you can’t do it. And ultimately, this former vice-president Dan Quayle, now in political winter in Arizona. rarely heard from on the American scene, talks to his fellow Indiana Republican and fellow vice-president Mike Pence and says, Mike, you can’t do it. You have to just count the votes.”

Why, after everything, after all the toadying, did Pence decide to do the right thing? “He’s someone who grew up as a Reagan Republican, a movement conservative, and that’s always been the core of who he is. He’s evolved during the Trump years to be a Trump Republican more than anything else.

“This is someone who at nearly every step for four years did what President Trump wanted him to do but, when it came down to the constitution, he could not break. There was the last temptation in his vice-presidency and he resisted but it was quite a journey to that moment of resistance.”

Hours after the rioters left the US Capitol building, Pence did certify the election result and, on 20 January, Trump skulked away from the White House as Biden was sworn in as the 46th president. But Trump’s “big lie” about a stolen election continues to spread, mutate and captivate the Republican party and a significant minority of the American population. Pence’s name now elicits boos from the “Make America Great Again” crowd.

Costa warns: “What happened in January 2021 was not a passing storm in American politics. It’s part of a new climate inside of the Republican party, a Republican party where in many states there is a demand to reject Biden’s victory, even to this day, and to elect Trump-allied people to election positions to make sure that President Trump, if he runs again, will have a smoother path to power.

“The lesson for so many Republicans after 2020, based on our reporting, is to do even more to make sure President Trump can come back and that comes through in the book in the narrative of Senator Lindsey Graham and others.”

Voting rights are under siege in Republican-controlled states. Pro-Trump state governors continue to prolong the pandemic by resisting mask and vaccine mandates. Last week Congressman Anthony Gonzalez of Ohio, one of 10 House Republicans who voted to impeach Trump, announced that he would not seek re-election, prompting the former president to gloat: “One down, nine to go!”

The battle for the soul of America, as Biden framed it, may only be at half-time. Costa continues: “American democracy has been tested over the past year and it will continue to be tested. President Trump is still out there on the political campaign trail.

“His rallies don’t get much coverage these days but, if you watch them, they have almost a war-like cadence in terms of the rhetoric. He sounds at times almost like Winston Churchill: we will never surrender, we will never give in.

“That’s the approach President Trump’s taking with his core voters, rallying them ahead of 2022 and the election in 2024. This is a political presence that does not want to go away and he is stoking his political capital at every turn, even if it doesn’t get much attention. He is on the road to political recovery inside the Republican party and perhaps even on the presidential campaign trail, if informally at this time.”

Towards the end of Peril, there is an account of a call between Trump and his former campaign manager, Brad Parscale, in early July this year. Parscale asked if Trump was going to run for president again. Trump replied: “I’m thinking about it. I’m really strongly thinking about running.”

Later, Parscale told others: “I don’t think he sees it as a comeback. He sees it as vengeance.”

Just the sort of word that Woodward and Costa could use for the title of their next book. If it comes to that.