One group that could yet emerge as the most powerful surrogates for Vice President Harris this year is a group of people who just four years ago thought they may never vote for a Democrat: Republicans who have turned against President Trump.

Elected officials at the state and national level, former aides to Trump and his family, and everyday voters who previously supported the 45th president captured the attention of Democrats at their convention this year. 

Just three speaking slots before Ms. Harris’s nomination acceptance address on Thursday was Congressman Adam Kinzinger — one of the ten GOP lawmakers who voted to impeach Trump after the January 6 riot at the Capitol. He decided against running for reelection in 2022. 

“I was a Republican for 12 years in Congress and I still hold on to the label. I never thought I’d be here, but listen — you never thought you’d see me here, did you?” he said, prompting a laugh from the audience. 

His call to Republicans was one for unity and a commitment to President Reagan’s values — values that he says the party has abandoned in the Trump era. “The Republican Party is no longer conservative. It has switched its allegiance from the principles that gave it purpose to a man whose only purpose is himself,” Mr. Kinzinger said. “Donald Trump is a weak man pretending to be strong.”

After Mr. Kinzinger’s speech, he was seen taking selfies with adoring Democratic volunteers backstage. 

On Wednesday, three prominent conservatives — all of whom knew Trump personally — came to the stage to say he could not be trusted with the presidency again. 

In one of the rare moments during a non-primetime speech where DNC attendees actually sat up in full attention, lieutenant governor of Georgia, Geoff Duncan, who was in office from 2019 to 2022, said Trump’s attacks on Georgia’s election in 2020 threatened his family’s safety, and that Trump could not be trusted in the White House against.

Mr. Duncan, who declined to run for reelection in 2022, considered running for president as an independent this year before deciding against it. 

“Tonight, I stand here as an American — an American who cares more about the future of this country than the future of Donald Trump,” Mr. Duncan said. “The only thing left to do is dump Trump! These days, our party acts more like a cult — a cult worshiping a felonious thug.”

He told the story of his young son who experienced protests outside of the Duncan home in the wake of the 2020 election, where Trump supporters raged against both Mr. Duncan, Governor Kemp, and other officials who refused to overturn Georgia’s election results. 

“My son came downstairs and he handed me this coaster that I had given him years before at a father-son retreat for our church, and he said: ‘hey dad, doing the right thing will never be the wrong thing. Stay strong,’” Mr. Duncan said, holding up a small piece of wood with the message written on it in sharpie. “To my fellow Republicans at home who want to pivot back toward policy, empathy, and tone: You know the right thing to do.”

Two former advisors to the Trump administration also appeared on Wednesday, saying that Trump was unfit for office. One, the former White House press secretary, Stephanie Grisham, marveled at the fact she was appearing at a Democratic convention, and made clear that she never supported Trump’s claims of election fraud. She served as White House press secretary for nearly a year between 2019 and 2020, and later became first lady Melania Trump’s press secretary and chief of staff until 2021. 

“I couldn’t be a part of the insanity any longer,” Ms. Grisham said, referring to her resignation from the White House on January 6, 2021, after the mob had breached the Capitol. “When I was press secretary, I got skewered for never holding a White House briefing. It’s because — unlike my boss — I never wanted to stand at that podium and lie. And now here I am, behind a podium, advocating for a Democrat.”

Another White House advisor, Olivia Troye, served as the homeland security and counterterrorism advisor to Vice President Pence. She resigned from her position in 2020 and publicly supported President Biden’s bid that year. On Wednesday, she came to Chicago to support Ms. Harris. 

“I saw how Donald Trump undermined our intelligence community, our military leaders, and ultimately, our Democratic process. Now, he’s doing it again,” Ms. Troye said. “What keeps me up at night is what happens if he gets back” to the White House, she added.

During the convention this year, Democrats played video testimonies from former Trump voters who said they had had enough of the former president, and that they would be voting for Ms. Harris this year. One older woman said she didn’t trust Trump to protect Social Security and Medicare, while another man said Trump’s character made it impossible to vote for him. 

One construction worker, Kyle Sweester, came to the stage to say that he had voted for Trump twice, but that his new proposals made it impossible to support him again. “He told us he’d look out for blue collar workers, so I made my first ever political donation to Trump,” the man said. “But then, I saw Trump’s tariff policy in action … I realized Trump wasn’t for me — He was for lining his own pockets.”