Robert F. Kennedy Jr., the vaccine skeptic running for president as an independent candidate, vowed to appoint a special counsel to investigate the Justice Department’s effort to prosecute those who rioted at the U.S. Capitol on Jan. 6, 2021, if he is elected, again downplaying the severity of the attack on the Capitol.

“I am concerned about the possibility that political objectives motivated the vigor of the prosecution of the J6 defendants, their long sentences and their harsh treatment,” Mr. Kennedy wrote in a statement a day after his campaign retracted a message of sympathy for the rioters.

A campaign email soliciting donations from supporters on Thursday said that Jan. 6 rioters jailed in Washington had been “stripped of their constitutional liberties.” But hours later, Stefanie Spear, the campaign’s press secretary, said the statement “was an error that does not reflect Mr. Kennedy’s views” and blamed a new marketing contractor, saying the relationship with the company had been terminated.

But on Friday, Mr. Kennedy said that “reasonable people, including Trump opponents, tell me there is little evidence of a true insurrection,” and repeated false claims that the rioters “carried no weapons, had no plans or ability to seize the reins of government.”

Video and other images have shown members of the mob wielding weapons like crowbars. Some have been charged or convicted of having guns and assaulting police officers with stun guns, pepper spray, baseball bats or improvised weapons like flagpoles. Far-right militiamen also amassed an arsenal of firearms near Washington in preparation for the attack, and some were convicted by juries of seditious conspiracy for their role in a plot to overturn the results of the 2020 election.

Mr. Kennedy’s statement on Friday aligned himself 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 have presented the Jan. 6 rioters as victims of zealous federal prosecutors.

Mr. Kennedy, whose political rise came from his promotion of vaccine misinformation and conspiracy theories about the government, has repeatedly downplayed the severity of the attack on the Capitol. In March 2023, when Mr. Kennedy was still a Democrat, he said on a podcast that members of his party had “an obsession” with the attack. In October, Mr. Kennedy said of Jan. 6: “What’s the worst thing that could happen? Right? I mean, we have an entire military, a Pentagon, a few blocks away.”

He has also repeatedly said this week that President Biden posed a greater threat to American democracy than Mr. Trump, and expressed sympathy for “people who say that the election is stolen” — counting himself as an example. He asserted in an interview 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.”