(CNN) — Former Vice President Mike Pence on Friday said he “cannot in good conscience” endorse presumptive GOP nominee Donald Trump, a stunning repudiation of his former running mate and the president he served with.

“Donald Trump is pursuing and articulating an agenda that is at odds with the conservative agenda that we governed on during our four years. That’s why I cannot in good conscience endorse Donald Trump in this campaign,” Pence said on Fox News.

The former vice president, after ending his own presidential bid in October, withheld an endorsement in the 2024 Republican primary, but he previously vowed to back the eventual GOP nominee. Trump had said after Pence dropped out that his former vice president should endorse him, saying, “I chose him, made him vice president. But … people in politics can be very disloyal.”

While he said he is “incredibly proud” of the record of the Trump-Pence administration, Pence argued that the former president has walked away from conservative issues, pointing to Trump’s stance on abortion and US national debt and his reversal on TikTok.

“During my presidential campaign, I made it clear there were profound differences between me and President Trump on a range of issues. And not just our difference on my constitutional duties that I exercised January 6th,” Pence said on “The Story with Martha MacCallum.”

“As I have watched his candidacy unfold, I’ve seen him walking away from our commitment to confronting the national debt. I’ve seen him starting to shy away from a commitment to the sanctity of human life. And this last week, his reversal on getting tough on China and supporting our administration’s efforts to force a sale of ByteDance’s TikTok,” he added.

Pence did not reveal who he’ll vote for in the 2024 general election, saying that he’ll keep “my vote to myself.” He, however, said he will “never vote” for President Joe Biden. Pence also suggested that he would not back a third-party candidate.

Pence broke with his boss in his decision to not overturn the 2020 election results while he was ceremonially overseeing Congress’ certification of Biden’s win on January 6, 2021. Pence later said Trump’s “reckless words” that day, when a mob of Trump supporters violently breached the US Capitol, “endangered” his family and others at the US Capitol.

In his own presidential campaign last year, Pence warned Republicans of the “siren song of populism” from Trump and his imitators.

Pence and his group, Advancing American Freedom, recently announced that they’re devoting $20 million to push conservative policies.

This story has been updated with additional reporting.