Former US vice-president Mike Pence has offered his most forceful repudiation of Donald Trump to date during a speech launching his 2024 campaign.
The Republican accused Mr Trump of being unfaithful to the US constitution and of abandoning conservative values.
The former Indiana governor and congressman formally commenced his White House campaign on Wednesday.
The move pits him against his two-time running mate, who he served under in the White House from 2017-21.
At a speech in Ankeny, Iowa, Mr Pence argued that Mr Trump had encouraged the mob that attacked the US Capitol on 6 January 2021, and had incorrectly asserted that Mr Pence had the power to overturn the election result.
"But the American people deserve to know that on that day, President Trump also demanded that I choose between him and the constitution," he said.
"Now, voters will be faced with the same choice: I chose the constitution and I always will."
He added that Mr Trump's actions on 6 January should disqualify him from returning to power.
"I believe that anyone who puts themselves over the constitution should never be president of the United States," he said.
"And anyone who asked someone else to put them over the constitution should never be president of the United States again."
Mr Pence, 64, also took shots at Democratic President Joe Biden as well, arguing that neither he nor Mr Trump are similar to average Americans who know how to "treat each other with kindness and respect even when we disagree".
Much of his speech focused on his disagreements with Mr Trump, but he also hit out at another Republican 2024 rival, Florida Governor Ron DeSantis.
"Donald Trump and others who would seek the presidency would walk away from our traditional role on the world stage," he said, hitting out at Mr DeSantis for his recent remarks about the war in Ukraine.
In a later CNN town hall, he continued attacking his rivals, critising Mr Trump for having previously called Russian President Vladimir Putin a "genius".
"I know the difference between a genius and a war criminal, and I know who needs to win in the war in Ukraine," he said.
But he also condemned prosecutors for investigating Mr Trump's alleged mishandling of classified documents after leaving office, saying "indicting a former president in the United States sends a terrible message to the world".
It comes after Mr Pence was cleared of wrongdoing last week by prosecutors over his own handling of classified info.
Mr Pence was one of eight Republican presidential candidates who appeared at a fundraising event at the Iowa state fairground, known as the Roast and Ride, last weekend.
He arrived on a Harley Davidson along with another couple of hundred bikers who rode in convoy, raising money for military veterans. But that was not enough to provoke much excitement around his candidacy.
An energetic Ron DeSantis got the most enthusiastic response as he promised to "fight the woke" wherever he finds it.
Mr Pence's speech, by comparison, was flat and lacklustre, perhaps demonstrating why a former vice-president with nationwide name recognition is trailing badly in opinion polls.
According to a Reuters-Ipsos survey released last month, Mr Pence is polling at 5%, compared to 44% for Mr Trump.
One former Trump voter, Todd Wilson, said he felt Mr Pence "needs to be a little bit stronger".
The candidate's humility was a plus, he said, "but I think if you're going to be the president, you got to have to have a little bit of a bulldog in you. So I'm looking for that out of Pence."
The Republican field for the 2024 nomination is increasingly crowded.
On Tuesday, former New Jersey Governor Chris Christie launched a bid. On Wednesday, North Dakota Governor Doug Burgum also announced that he was entering the race.
Mr Pence was seen as an unswervingly loyal deputy to Mr Trump through much of their four years in office.
But he has distanced himself since the riot at the US Capitol.
Mr Trump pressured Mr Pence to overturn Joe Biden's election victory when he presided over the certification of results in Congress and admonished him for lacking "courage" when he refused.
Some rioters were heard chanting "hang Mike Pence" as they stormed the halls of Congress, and many Trump loyalists view him as a traitor.