NATE POWELL calls it “the most intensive thing I’ve ever done for a book cover.” Yet every step in a the five-month process, he says, was worth it.

The art in question is the cover of the forthcoming “March: Book Three” (IDW/Top Shelf), the latest installment in the RFK Award-winning trilogy and civil-rights memoir by Rep. John Lewis (co-authored by Andrew Aydin). The cover depicts the 1965 violent showdown in Selma, and today, on the 51st anniversary of that “Bloody Sunday,” The Post’s Comic Riffs offers the exclusive first “sneak peek” at that art.

On the literal level, the cover depicts hundreds of nonviolent protesters led by Lewis coming face to face with Sheriff Jim Clark’s officers, Alabama State Trooper John Cloud’s men, and other armed authorities. But part of the painstaking creative process was making sure this scene also worked as an apt representation of the larger fight for rights.

“March: Book Three,” which is set to be 250 pages (“I’ve only got 90 more” to draw, Powell notes), promises to be a sweeping epic, opening with the 1963 Birmingham church bombing that killed four young girls, and moving through the post-JFK-assassination events of 1964-65, culminating with LBJ’s summer signing of the Voting Rights Act.

And Rep. Lewis (D-Ga.) calls “Bloody Sunday” the “apex of the movement” that chastened a nation and hastened the voting legislation. And so for Powell, the Indiana-based artist, this meant that the Selma scene had to precisely reflect larger themes and context. “This is about a movement as an organization, coming together,” Powell says.

There were three main versions of this cover, and Powell worked closely with Lewis, Aydin, Top Shelf/IDW editor Leigh Walton and designer Chris Ross to balance the needs of both documentary and narrative. That meant employing forced perspective and the repositioning of bodies in the frame, including moving some of the photographers and state troopers.

“This was a very positive experience in active listening,” says Powell, whose other acclaimed books include “Swallow Me Whole.” “The process of compromise was a great learning experience.”

“With the cover of the third book,” Powell says, “you’re capturing a massive social movement from being true to the perspective of the primary storyteller who was at the center of the movement” — but this is all set against the larger swirl of events beyond Lewis’s sight lines. And then there are aesthetic choices that are informed by, but don’t stick literally to, still and video images from that day. “I had to stick up, for both the subjective and the objective,” the artist says.

And unlike the previous books in the series, the title “March” no longer bifurcates multiple images on the cover; instead, the word assumes the contours of the road. “Because,” Powell says, “the march is now on the streets.”