Donald Trump’s promise to pardon supporters who attacked the US Capitol on January 6 2021 was “the stuff of dictators”, Richard Nixon’s White House counsel warned.

Trump made the promise at a rally in Conroe, Texas, on Saturday.

“If I run and if I win,” he said, referring to the 2024 presidential election, “we will treat those people from January 6 fairly. We will treat them fairly. And if it requires pardons, we will give them pardons because they are being treated so unfairly.”

More than 700 people have been charged in connection with the Capitol attack, around which seven people died as Trump supporters tried to stop certification of his election defeat, in service of his lie that it was caused by electoral fraud. More than 100 police officers were hurt.

Eleven members of the far-right Oath Keepers militia have been charged with seditious conspiracy. Trump himself was impeached for inciting the riot. Ten House Republicans voted to impeach but Trump was acquitted when only seven Republican senators found him guilty. That left him free to run for office again.

John Dean, 83, was White House counsel from 1970 to 1973 before being disbarred and detained as a result of the Watergate scandal, which led to Nixon’s resignation in 1974. Dean responded to Trump on Twitter.

“This is beyond being a demagogue to the stuff of dictators,” he wrote. “He is defying the rule of law. Failure to confront a tyrant only encourages bad behaviour. If thinking Americans don’t understand what Trump is doing and what the criminal justice system must do we are all in big trouble!”

Trump was generous with pardons in office, recipients including Steve Bannon and Michael Flynn, both now targets of the House committee investigating January 6 and with Trump in its sights. On Sunday morning, the New Hampshire governor Chris Sununu, widely seen as a relative moderate in Trump’s Republican party, was asked if pardons should be offered to Capitol rioters.

“Of course not,” he told CNN’s State of the Union. “Oh, my goodness. No.”

Even Lindsey Graham, the South Carolina senator and dogged Trump ally, said the former president was wrong.

“I don’t want to send any signals that it was OK to defile the Capitol,” he told CBS’s Face the Nation. “I want to deter what people did on January 6, and those who did it, I hope they go to jail and get the book thrown at them because they deserve it.”

But a moderate Republican, Susan Collins of Maine, indicated the hold Trump has on the party.

Appearing on ABC’s This Week, the senator said Trump should not “have made that pledge to do pardons. We should let the judicial process proceed.”

But Collins, who voted to convict Trump over the Capitol attack, would not say that she would not support him if he ran for president again.

“Well certainly it’s not likely given the many other qualified candidates that we have, that have expressed interest in running,” she said. “So it’s very unlikely.”

Trump dominates polling concerning potential Republican nominees for 2024.

Others deplored Trump’s words in Texas. Richard Painter, a White House ethics counsel under George W Bush, said the promise of pardons should, constitutionally speaking, stop Trump running again.

“This alone is giving aid or comfort to an insurrection within the meaning of the 14th amendment, section three,” Painter wrote. “Trump is DISQUALIFIED from public office.”

Trump also complained about investigations of his business and political affairs which have landed him legal jeopardy. On Sunday, Graham, whose actions in support of Trump are being investigated in Georgia, said he would cooperate if asked.

“Yeah,” he said. “Give me a call.”

But he also complained about a supposed “effort here to use the law, I think inappropriately. So I don’t know what they’re going to do in Fulton county [Georgia]. I don’t know what the January 6 committee is going to do. I expect those who defile the Capitol to be prosecuted. But there’s a political movement using the law to try to knock Trump out of running. And I, particularly, don’t like it or appreciate it.”

In Texas, Trump urged supporters to protest.

“If these radical, vicious, racist prosecutors do anything wrong or illegal,” he said, “I hope we are going to have in this country the biggest protests we have ever had in Washington DC, in New York, in Atlanta and elsewhere, because our country and our elections are corrupt.”

Prosecutors, he said, were “trying to put me in jail. These prosecutors are vicious, horrible people. They’re racists and they’re very sick. They’re mentally sick. They’re going after me without any protection of my rights by the supreme court or most other courts.”

Glenn Kirschner, a former federal prosecutor now a legal analyst for NBC, said: “Trump is not only encouraging his supporters to violence if he’s arrest[ed], he’s also signaling that he’ll pardon them, just as he’ll pardon the [January 6] insurrectionists.

“Will this finally move prosecutors to hold him accountable for his crimes?”