ANKENY, Iowa — Vice President Pence opened his bid for the Republican nomination for president Wednesday with a firm denunciation of President Trump, accusing his two-time running mate of abandoning conservative principles and being guilty of dereliction of duty on January 6, 2021.

On that perilous day, Mr. Pence said, as Mr. Trump’s supporters stormed the Capitol after the president insisted his vice president could overturn the election results, Mr. Trump “demanded I choose between him and our Constitution. Now voters will be faced with the same choice.”

Mr. Pence is the first vice president in modern history to challenge the president under whom he served. While he spent much of his speech, delivered at a community college in a suburb of Des Moines, criticizing Democratic President Biden and the direction he has taken the country, he also addressed January 6 head-on, saying Mr. Trump had disqualified himself when he declared falsely that Mr. Pence had the power to keep him in office.

Mr. Trump’s statements about mass voting fraud led a mob of his supporters to storm the Capitol, sending Mr. Pence and his family scrambling for safety as some in the crowd chanted, “Hang Mike Pence!”

“I believe anyone that puts themselves over the Constitution should never be president of the United States, and anyone who asks someone else to put them over the Constitution should never be president of the United Sates again,” the former vice president said.

Mr. Pence has spent much of the past two-and-a-half-years grappling with fallout from that day as he has tried to chart a political future in a party that remains deeply loyal to Mr. Trump and is filled with many who still believe Mr. Trump’s claims that the 2020 election was stolen and that Mr. Pence somehow could reject the results.

While Mr. Pence has criticized Mr. Trump as he has worked to forge an identity of his own outside the former president’s shadow, he has generally done so obliquely, reflecting Mr. Trump’s continued popularity in the party. But Wednesday, as Mr. Pence made his pitch to voters for the first time as a declared candidate, he did not hold his tongue.

He accused the former president of abandoning the conservative values he ran on, including on abortion.

Mr. Pence, who supports a national ban on the procedure, said, “After leading the most pro-life administration in American history, Donald Trump and others in this race are retreating from the cause of the unborn. The sanctity of life has been our party’s calling for half a century — long before Donald Trump was a part of it. Now he treats it as an inconvenience, even blaming our election losses in 2022 on overturning Roe v. Wade.”

Mr. Trump has declined to say what limits he supports nationally and has blamed some midterm candidates’ strong rhetoric for their losses last November.

Mr. Pence also bemoaned the current politics of “grudges and grievances,” saying the country needs leaders who know the difference between the “politics of outrage and standing firm.”

“We will restore a threshold of civility in public life,” he pledged

Nonetheless, Mr. Pence did not rule supporting Mr. Trump if the former president wins the GOP nomination.

“I will absolutely support the Republican nominee in 2024, especially if it’s me,” Mr. Pence said on Fox News Channel after his announcement.