John Lewis, the civil rights leader turned 17-term congressman from Georgia, has seen plenty in his 80 years that would test one’s faith in the long fight.

As a young man, Lewis, the third of 10 children from a sharecropping family in Alabama, led sit-ins to integrate lunch counters in Nashville, marched with Martin Luther King, and was beaten by the Klan as part of the Freedom Riders in the South Carolina; he’s been arrested over 40 times, five as a legislator, and spent months in jail.

As congressman, he repeatedly defended the necessity of his signature civil rights achievement – the Voting Rights Act of 1965, for which he had his skull fractured by police crossing the Edmund Pettus Bridge on Selma’s Bloody Sunday – only to see it stripped of its enforcement power by the supreme court in 2013, and further gutted as Republican legislatures enacted voter suppression laws in the last decade.

He’s witnessed the election of Donald Trump, whom he has called “without a doubt” the “worst president for civil rights since the 1960s”.

Now, as America convulses with the largest and most widespread protests for racial justice since the 1960s civil rights movement, as corporate America and cultural institutions reckon with their roles in systemic racism and segregation, as Black Lives Matter activists direct attention to how much has not changed since the days of Jim Crow legislation, a new film revisits and celebrates Lewis’s long fight.

John Lewis: Good Trouble, directed by Dawn Porter in partnership with CNN films, a by-the-books documentary on a storied and heroic subject, ties Lewis’s oft-romanticized civil rights activism in the 1960s with his role as the “conscience of Congress” in over 30 years in the House. The film’s title derives form Lewis’s slogan (and, in recent years, hashtag) of getting in “good trouble” – non-violent agitation to direct attention to injustice, or to the shoddy foundation of the rules.

Good Trouble toggles between Lewis’s work in the cauldron of the civil rights movement of the early 1960s and on the road in 2018, as he campaigned for Democratic candidates in the midterm elections. Odes to Lewis often immediately reach back to his work in the 1960s, but footage from the campaign trail – Lewis bonding with an older black woman in Georgia about picking cotton as children, or invoking a crowd in Texas to get in good trouble with the steady passion of a seasoned preacher – demonstrate how “he’s still doing the work,” Porter told the Guardian. “That is not ancient history. It’s very much our present.”

Lewis’s present, at least before his diagnosis with stage four pancreatic cancer in December 2019, is as a senior leader in the House of Representatives, and a role model and mentor for members of a diverse, progressive freshman class including Alexandria Ocasio-Cortez, Ilhan Omar, Rashida Tlaib and Ayanna Pressley, all of whom testify to Lewis’s influence in the film (along with such veterans as Hillary and Bill Clinton, Nancy Pelosi and the late Elijah Cummings).

“If John Lewis at 19, 20 wasn’t doing what he did, I wouldn’t be here today,” says Ocasio-Cortez.

Lewis’s roots are not as a legislator so much as an agitator steeped in the principles of non-violence as a philosophy preached by the Rev James Lawson, whom Lewis discovered as a young man at Fisk University in Nashville. Good Trouble traces Lewis’s bone-deep commitment to non-violence as a practice – as a student activist in the Nashville sit-ins, as one of the 13 original Freedom Riders crisscrossing the south to expose the federal government’s passivity in enforcing desegregation of buses, as co-founder and chairman of the Student Nonviolent Coordinating Committee (SNCC) – from the early 1960s to the present, such as his leading role in a 26-hour sit-in at the Capitol to demand gun control legislation after the 2016 Pulse nightclub massacre in Orlando.

In an echo of renewed commentary on the necessity of violence to command attention for change, Good Trouble revisits conflict within the fracturing 1960s civil rights movement, as Lewis was hesitant to cheer Black Power and ceded leadership of the SNCC to Stokely Carmichael, who considered non-violence a tactic rather than principle.

In the film, Lewis recalls solidifying his desire to run for office upon the assassination of Robert F Kennedy, after which he experienced a depression, and “worried about what was happening in America”, according to Porter.

“And I think when he came out of that, he took a long, hard look at himself,” what he has called an “executive session”, “and he decided that he had seen horrific things but he had also seen amazing power and strength in the collective humanity, and so that was the path he was going to follow and be guided. And I think he has not looked back from that.”

Throughout Good Trouble, and in an interview this month with New York magazine in the wake of nationwide protests sparked by the killing of George Floyd by the Minneapolis police officer Derek Chauvin, Lewis displays an unflappable optimism in the path forward, an unwavering belief in non-violence as principle, faith in “the conscience of the American people” when confronted and the ballot as “the most powerful non-violent tool we have in a democratic society.

The steadiness never wavered behind the scenes, either, according to Porter. Over the course of filming in 2018 and early 2019 – years in which a good, oh, 50 or so dispiriting, democracy death-spiral news cycles one-upped themselves – Lewis guided her “away from being reactive back to a proactive, ‘What do you want to accomplish? What do we need to do?’” she recalled.

“After a while, I started to get the message, which is that if a John Lewis at 80 years old is optimistic and still believes in the goodness of people, that we ultimately will get to a place where there’s more members of a beloved community, that means something,” Porter said. “He’s seen so much in his life that could make him bitter. And that is just not who he is.”

“He really focuses on a legislative opportunity, but he also thinks there’s a morality in government that should be part of the conversation,” Porter said. But she hoped the film, in which her camera finds Lewis joking with his sisters, talking with too many people in the airport and showing off a collection of ceramic chickens, shows Lewis’s “very subtle, wry humor”.

“He is a very peaceful person. He is a person who has seen horrific things, but he can also appreciate life in its fullness.”

Lewis frequently makes the call for Good Trouble but admits, in the film, to a deep fear: that one day he will wake up, and American democracy will be gone. It’s a fair question, given the flagrant transgressions and dog-whistling of the current administration, if that point is nearing, or gone. How would we know? “We’ve come too far, we’ve made too much progress, to slow down or to go back,” Lewis told New York in response to the fear. “So we must go forward.”

Lewis’s public appearances have waned in the wake of his cancer diagnosis. But as he told Porter, staring unbowed at the camera: “As long as I have breath in my body, I will do what I can.”

• John Lewis: Good Trouble is available digitally in the US on 3 July with a UK date yet to be announced