The presidential campaign of Robert F. Kennedy Jr., the vaccine skeptic running as an independent candidate, said in an email on Thursday that rioters charged with crimes in connection with the Jan. 6, 2021, attack on the Capitol and held in a Washington jail had been “stripped of their constitutional liberties.”

Nearly four hours later, the campaign retracted the statement. Stefanie Spear, the press secretary for Mr. Kennedy’s campaign, said the statement “was an error that does not reflect Mr. Kennedy’s views,” adding, “It was inserted by a new marketing contractor and slipped through the normal approval process.”

Ms. Spear said that the campaign was reviewing the contract with the marketing company, and stressed that “anybody who violated the law on Jan. 6 should be subject to appropriate criminal and/or civil penalties.”

The email appeared to align the candidate closer to the position of former President Donald J. Trump, one of his opponents in the presidential race, as well as others on the right who say the rioters have been treated unfairly by the justice system — even painting them as martyrs. It follows comments from Mr. Kennedy this week suggesting that President Biden posed a greater threat to American democracy than Mr. Trump.

The Kennedy campaign email, soliciting donations from supporters, focused mostly on the plight of Julian Assange, the embattled WikiLeaks founder who is facing extradition to the United States to face charges of violating the Espionage Act. The campaign described Mr. Assange as a “political prisoner” and asked supporters to sign a petition urging the United States to drop the charges against him.

“The Brits want to make sure our government doesn’t kill Assange,” the email said, before adding that “this is the reality that every American citizen faces — from Ed Snowden, to Julian Assange to the J6 activists sitting in a Washington, D.C., jail cell stripped of their constitutional liberties.”

Mr. Trump and allies in Congress, as well as far-right circles online, have presented the Jan. 6 rioters as heroes and martyrs, falsely suggesting either that the riot was a peaceful protest against voter fraud or that the violence they carried out on that day was necessary to overturn the results of the election and install Mr. Trump in office for another term. Particular attention has been paid to those rioters who have been held at the local jail in Washington.

Mr. Trump has made this revisionist view of the events of Jan. 6 central to his campaign. He has vowed to pardon those facing charges over their participation in the attack at the Capitol, and he has at recent rallies referred to those detained in connection with their participation as “hostages.”

Mr. Kennedy, who has founded his political career on promoting vaccine misinformation and conspiracy theories about the government, has described Mr. Trump’s efforts to overturn the election as “appalling,” but he has also expressed sympathy for “people who say that the election is stolen” — counting himself as an example. He asserted in an interview on CNN on Monday that the 2000 and 2004 presidential elections were stolen from the Democrats, adding, “We shouldn’t make pariahs of those people, we shouldn’t demonize, and we shouldn’t vilify them.”

Mr. Assange and Mr. Snowden have at times received sympathy from both liberal and conservative figures, as well as nonpartisan press freedom advocates, who say they are the victims of overzealous prosecution in violation of the constitutional rights to free speech and freedom of the press. Mr. Trump, who once sided with Mr. Assange over the conclusions of America’s intelligence services, had considered pardoning both Mr. Assange and Mr. Snowden — who fled into exile in Russia more than a decade ago — during his term in office.

The Kennedy campaign’s grouping the two men, wanted for leaking U.S. intelligence secrets, with the rioters who stormed the Capitol stood out — particularly the reference to the Washington jail.

More than 1,250 people have been charged with crimes in connection with the attack — and hundreds of people have been convicted. But only 29 defendants are being held in the Washington jail, according to a review of the cases by NBC News, and most have already been convicted of violent crimes, including assaulting police officers during the attack. NBC also reported that just 15 Jan. 6 defendants were still in pretrial detention.

Michael Gold contributed reporting from New York.