LAST YEAR, on a warm April day, Rep. John Lewis sat on stage at D.C.’s Walter E. Washington Convention Center and shared the story of what new worlds he had encountered since becoming Congress’s first-ever graphic novelist. “I went to Comic-Con and I’ve been to Small Press [Expo] … and Dragon Con,” the congressman recounted about attending his first nerd-culture festivals. “It sort of changed my life.”

Just then, this living legend hopped for a half-second and pivoted toward me, to reveal just how much he’d observed at these events. I was moderating the congressman’s panel at Awesome Con, to celebrate “March: Book One” — the launch of Lewis’s acclaimed civil-rights trilogy as illustrated memoir — and had introduced him as the truest hero at an event packed with people “cosplaying” (or costume-playing) as fictional caped crusaders and superpowered crimefighters.

“The mistake I made, when I went out to Comic-Con, I didn’t wear my official outfit,” said the ever-dapper Georgia congressman, in coat and tie, nodding to the popularity of cosplay at Comic-Con International in San Diego, the American granddaddy of such pop-culture circuses. I decided to bite.

“–And what is that outfit?” I interjected, following the congressman’s lead as if we’d scripted it. His eyes twinkled.

“Well, I thought I should have a backpack on,” said Lewis, then 73, as the crowd laughed at his spirit of sartorial playfulness.

“I should have had my trench coat on. And maybe a cap — just maybe,” continued Lewis, glancing left toward Andrew Aydin, his young staffer and “March” co-author, who, as a lifelong comics geek, had introduced his boss into this very world. (It had been Aydin’s idea in the first place to urge Lewis to write a graphic-novel memoir — a conversation born out of fellow staffers having teased Aydin several years earlier for heading out to Comic-Con. Now, we had come full circle.)

“But the people have been amazing — they’ve been wonderful,” continued Lewis, appreciating the open-armed response he’d received while attending comics events. At “Comic-Con, and just moving around America — the children, the young people.”

In that convention-center room, on that day, we’d thought the congressman was kidding about the costume. But I should have known better from that gleeful glint in his eye: Rep. Lewis had filed that idea away.

And so, on Saturday, the civil-rights hero made his return to Comic-Con, this time to support “March: Book Two” alongside Aydin and the trilogy’s artist, the Eisner Award-winning Nate Powell. And this time, Lewis indeed chose to cosplay.

Trench coat? Check.

Backpack? Check.

And inside that backpack were even a couple of books: the first two volumes of the RFK Award-winning “March.”

Upon landing in San Diego, Lewis had decided to dress up as his 25-year-old self — the young man who, on the road to Selma, Ala., led 600 civil-rights marchers peacefully across the Edmund Pettus Bridge, at the foot of which the nonviolent protestors were beaten and tear-gassed by state troopers, many on horseback. Attacked by nightstick, his skull fractured, Lewis bled that day for the cause. He told me: “I saw death. I thought I was going to die.”

The March 1965 incident would become known as “Bloody Sunday,” and would help turn the nation’s sociopolitical and moral tide, as later that year, President Johnson signed the Voting Rights Act.

And a half-century later, John Lewis — the man who had launched himself into a life of nonviolent protest after reading a Martin Luther King comic book as a teenager, and who, by age 23, was the youngest speaker at the March on Washington — would again march over the Edmund Pettus Bridge, this time arm in arm with the nation’s first black president.

Now, in San Diego, it was time to march again. But how to pull together the congressman’s “costume” in time?

“We went looking for the [type of] trench coat I wore 50 years ago,” Rep. Lewis tells me over the weekend. They found, too, an accurate backpack.

“In his backpack were two books, an apple, a toothbrush and toothpaste,” Aydin tells The Post’s Comic Riffs. “He carried that in Selma 50 years ago in case he got arrested.”

Also in John Lewis’s backpack on that day in Selma was an orange. “We tried but couldn’t find an orange,” Rep. Lewis tells me of completing his new weekend costume. “In Southern California, of all places.”

And there, as the three “March” collaborators arrived at their Saturday panel, was not just a large, eager audience, but a grouping of schoolchildren down in front. Lewis told Comic Riffs two years ago that a motivating factor in writing “March” as a graphic novel was to reach the next generation — to have them, through art, intimately witness the emotional and physical toll that the path to freedom took, from sit-ins to freedom rides.

Now, sitting here, were young fans of “March” — including about two-dozen bright-eyed third-graders from nearby Oak Park Elementary, a Title I school. Their teacher, Mick Rabin, had shown the wisdom to bring them to meet history in the flesh. (Lewis is the last living member of the “Big 6” civil-rights leaders.)

Rabin, a UCSD alumnus, grew up in San Diego as a diehard comics fan. He liked war stories, notably the works of Joe Kubert, he tells me, and he devoured issues of “Sgt. Rock.” And he first attended Comic-Con in 1975, when the event, like Rabin, was still a pup — drawing only several thousand people over a weekend. (The event now attracts more than 125,000 over four days and five nights.)

“And I’ve been to almost every Comic-Con since 1982,” Rabin tells The Post’s Comic Riffs, underscoring his fandom of not only of the art form, but also this convention.

Rabin brings that same passion to the classroom, actively seeking ways to teach that don’t succumb to “the drudgery,” he says. In that spirit, he introduced his students to Lewis’s “March” graphic novels, and their interest in reading was piqued.

“People incorrectly underestimate the ‘readiness’ that young students have for complex subjects like civil rights or LGBT safety and inclusion. But that’s patronizing,” Rabin tells me. “They can handle it. It just takes a bit more time, effort, and front-loading. Third-graders can really relate to social justice, because nobody in the world comprehends the idea of what is ‘fair’ better than an 8-year-old.  And ‘fairness’ is just another way of saying ‘social justice.’ Kids can and do really get it.”

With that belief in grade-schoolers in mind, why not take them to the same event that Rabin attended as a boy, and have them meet the congressman himself? Who knew just what that meeting might bring?

Rabin worked out the logistics, managed to score about a dozen comp passes, and come the morning of that Saturday panel, Lewis’s young fans were front and center.

And then, after the talk, Lewis needed to get back to the booth of his publisher, Top Shelf Productions, to sign books. Yet how to handle having so many young fans in tow?

The only true answer, of course, was to march.

Rabin says he proposed that his students walk with their living hero.  “So we marched through the Convention Center to the showroom floor and the Top Shelf booth,” Aydin recounts. And as they did, the procession swelled, as some attendees joined in the parade of purpose.

And leading the way was John Lewis, once again a true hero in a hall full of Spandex’ed superheroes, holding a child’s hand in each of his.

“This is the most meaningful con I’ve ever attended for that alone,” Rabin tells Comic Riffs. “I know that the students will never forget it, either.”

And what did the moment mean to the congressman himself?

“I felt very, very moved just by being with the kids,” Lewis tells me on Sunday, shortly before making a community visit in San Diego. “As you know, the civil-rights movement was often led by the children, and the young people.”

And by putting on a trench coat, it seemed as if John Lewis, the costumed Selma marcher, was peeling back the years.

“It just felt special — I was in the moment,” John Lewis says. “It felt like I was living a portion of my life all over again.”