A boy recently received a Christmas card signed by more than two dozen of his heroes. Many of them took the time to add small notes reflecting the sentiments of the season.

“Have a jolly Christmas!”

“Merry Christmas! #45 Won.”

“Merry Christmas Patriot!”

“Let’s Go Brandon!”

The boy’s name is Joshua, not Brandon. (The Brandon reference is code for a vulgar put-down of President Biden.) And the card did not come from the boy’s favorite sports team, but from a detention center in Washington, where these holiday well-wishers await trial on charges stemming from the Jan. 6 attack on the Capitol.

A photo of the Christmas card recently appeared on a website that portrays the Jan. 6 defendants as wrongly imprisoned patriots. It is both a seasonal greeting and a political artifact — a reflection of their status as symbolic martyrs who embody the beliefs and resentments now ascendant within the Republican Party, animated by former President Donald J. Trump’s baseless assertion that the 2020 election was stolen from him.

The recasting of the self-described “1/6ers” as American-style Nelson Mandelas is playing out in the courts, in Congress and online. Stephen K. Bannon, a former adviser to Mr. Trump, often asks listeners of his podcast to support the legal defense of these “political prisoners,” while Tucker Carlson, the Fox News host, recently produced a streaming special, “Patriot Purge,” that suggested the defendants were victims of a “false flag” operation orchestrated by their own government.

The anniversary of the Jan. 6 riot is two weeks away, but the Justice Department’s sprawling investigation continues apace, with more than 700 arrests for crimes ranging from misdemeanor trespassing to felony assault. The roughly 50 defendants sentenced so far have received little or no prison time, with notable exceptions.

The defendants still in custody generally face the most serious charges, including conspiracy to obstruct the operations of Congress and assaulting police officers — although some of those detained have not been accused of violence.

With trials related to the Capitol riot scheduled throughout the contentious midterm election year of 2022, how the Jan. 6 defendants are perceived — particularly the three dozen held for months in the Washington jail — may become another divisive campaign issue.

A stark example of the martyrdom effort is a drawing by Jon McNaughton, a Utah illustrator who specializes in demonizing Democrats (Speaker Nancy Pelosi as a witch) and celebrating Republicans (Mr. Trump as Mount Rushmore-worthy). It depicts a shivering Jan. 6 inmate on a gray cell floor, his bare feet shackled, his face obscured by a red MAGA hat.

These were not quite the conditions that Representative Marjorie Taylor-Greene, a Republican provocateur from Georgia, found during a November visit to the Washington jail’s so-called Patriot Wing, where the detainees are held. No bare feet, no shackles. But in a follow-up report called “Unusually Cruel,” she asserted that the detainees were being kept in “inhumane” conditions and had been denied basic needs because of their political beliefs.

Ms. Taylor-Greene’s report makes no effort to mask her sympathies for the defendants, noting that she and Representative Louie Gohmert, a Texas Republican, signed their Bibles and copies of the Constitution. At the end of her tour, she gathered the inmates in a circle and led them in prayer.

Around the same time as Ms. Taylor-Greene’s visit, two of the inmates, Ryan Nichols and Robert Morss, sent a federal judge a list of 77 grievances about a facility they described as a “pale dungeon of human rights violations.”

Both Mr. Nichols, a former Marine from Texas, and Mr. Morss, an Army veteran from Pennsylvania, are charged with being at the fore of the assault on police officers protecting the Capitol. Mr. Nichols was recorded on video shouting through a bullhorn, “If you have a weapon, you need to get your weapon!”

Now, from prison, the two men claimed that they had been force-fed critical race theory propaganda and left to beg for water, medical aid and mercy. The only thing not taken from them yet, they wrote, “is our souls which lay within the weakened bones of our famished chests.”

Washington’s jails have long been a source of local shame and outrage. The U.S. Marshals Service recently reported finding sewage and water leaks at the jail, as well as instances of corrections officers withholding food and water as punishment. But the more extreme conditions were not in the section of the jail where the Jan. 6 defendants are held.

Karl A. Racine, Washington’s attorney general, said last month that the squalid conditions in the jail — where most inmates are Black — had “received little attention until they were raised by mostly white defendants accused of perpetrating the Jan. 6 insurrection.”

In court filings and accounts shared by supporters, though, the incarcerated Jan. 6 defendants have used these conditions to augment their portrayal of themselves as oppressed but resolute prisoners of war. Every night at 9 p.m., they stand to salute an American flag and sing “The Star-Spangled Banner.”

But there have been cracks in this solidarity.

Steven A. Metcalf, a lawyer who represents two of the inmates — Edward Lang, a self-described social media influencer from Newburgh, N.Y., and Dominic Pezzola, a member of the far-right Proud Boys from Rochester, N.Y. — said he believed the government was purposely keeping the accused rioters together. “To see who is a leader, who is a follower and who might cooperate,” he said.

But Mr. Metcalf said that an unhealthy environment had developed, with inmates in the “Patriot Wing” growing suspicious of who among them might be a government informant. He said that when one defendant mentioned another in a court filing, a fistfight broke out.

Questions have also arisen about creating a hothouse of sedition by keeping together like-minded inmates who are charged with attacking the seat of government.

In October, a lawyer for Thomas Sibick — a Buffalo, N.Y., man facing charges that include stealing an officer’s badge and radio — told a federal judge that his client wanted to escape the Jan. 6 unit’s “almost cultlike” environment so much that he had opted for solitary confinement. The judge released Mr. Sibick to the custody of his parents, in part because she worried that the unit’s “toxic environment” might lead to further radicalization.

Last month another accused rioter, Robert Gieswein, also sought his release by citing the jail’s supposed “Groundhog Day” dynamic. A self-proclaimed militia member from Colorado, Mr. Gieswein had come to Washington in January talking about ridding the Capitol of all politicians beholden to the Rockefellers and the Rothschilds.

The judge denied Mr. Gieswein’s request for freedom. Shortly afterward, the inmate signed the Christmas card to the boy named Joshua as “Machine Gun Bobby.”

A photograph of the Christmas card is featured on the Patriot Freedom Project website, which was created by Cynthia Hughes, an aunt of one of the Jan. 6 prisoners, Timothy Hale-Cusanelli, described by prosecutors as an “avowed white supremacist and Nazi sympathizer.”

The website managed by Ms. Hughes, who declined to comment, provides links so people can donate to defense funds and offers ways to “adopt a ⅙ family” or write letters of support to any of the inmates. Merchandise is also for sale, including “Due Process Denied” yard signs and “Free the ⅙ers” hoodies.

In a short note accompanying a photograph of the Christmas card, Ms. Hughes said that a couple and their three children sent letters and drawings to “our J6ers” every day “just to put a smile on their faces.” The Jan. 6 inmates showed their gratitude, in turn, by sending back Christmas cards.

The cards were signed by, among others, Cleveland Meredith Jr. of Georgia, charged with threatening to shoot Ms. Pelosi and Mayor Muriel Bowser of Washington in the head; Alan Byerly of Pennsylvania, charged with menacing officers with a Taser and assaulting a news photographer; and Kyle Fitzsimons, a butcher from Maine who wore a white apron as he racked up 10 criminal charges, including assault.

Some who signed the card took pains to include their nicknames. In addition to “Machine Gun Bobby,” there were “So-Cal Patriot” (David Dempsey, a Los Angeles-area Proud Boy), charged with spraying officers with a chemical agent and using a metal pole and a crutch as weapons; “The Ginger Ninja” (Shane Jenkins), charged with using several objects — including a flagpole, a metal rod, and a desk drawer — to attack officers; and “Gator 1” and “Gator Six” (Kelly Meggs and Kenneth Harrelson), accused of being part of an Oath Keepers militia contingent intent on searching for Ms. Pelosi and disrupting the certification of the election.

Other defendants managed to keep politics out of Christmas in their holiday wishes to the boy. Among them: Peter Stager, a long-distance truck driver from Arkansas facing several felony charges, including that he used a pole bearing the American flag to beat a police officer being dragged down the outdoor steps of the Capitol.

All Mr. Stager wrote was: “Have a Merry Christmas and God bless your house.”