Carmel, Indiana - Former vice president Mike Pence said Friday he would not be backing Donald Trump as his old boss runs for a second term in the White House.

"It should come as no surprise that I will not be endorsing Donald Trump this year," Pence said in an interview with Fox News.

US media called the announcement a "bombshell" and "startling," although in reality deep divisions have driven the two men apart since leaving office, and an endorsement would have been a surprise.

The pair became estranged after Trump tried to pressure Pence to help him overturn his 2020 election defeat to Joe Biden, repeatedly attacking him on social media when he wouldn't go along with the scheme.

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After various attempts by Trump and his allies to subvert the election results failed, the then-president directed a mob of his supporters to march on the Capitol, where they ransacked the building as some chanted "Hang Mike Pence!"

Pence told Fox News that Trump was "pursuing and articulating an agenda that is at odds with the conservative agenda that we governed on during our four years."

The 64-year-old's comments come days after Trump secured enough delegates to clinch the Republican nomination to challenge Biden in November.

Pence was one of Trump's early rivals in the primary contest ahead of the 2024 election, although he quit the race last October after failing to poll in double figures.

Pence did not offer an alternative endorsement – there are several third-party candidates running – and said he would never vote for Biden, who secured the Democratic nomination this week.

Pence gave Trump years of unswervingly loyal service before the insurrection in Washington.

But he called the president's actions on January 6, 2021, "reckless" and said they "endangered me and my family."

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Trump had demanded Pence, a former governor of Indiana, to derail the congressional certification of Biden's electoral victory.

Pence refused, gaining the enmity of Trump's diehard followers and becoming persona non grata within the hard right "Make America Great Again" movement.

The evangelical Christian also received unwelcome headlines after leaving office over an FBI search of his home – part of a wider Washington scandal over mishandled classified documents – and a subpoena to testify before a federal probe into the Capitol riot, which he resisted.

He defended the record of the 2017-21 Trump administration in his Fox News interview, saying he was "incredibly proud" of its achievements.

"It was a conservative record that made America more prosperous, more secure, and saw conservatives appointed to our courts in a more peaceful world," he said.

"But that being said, during my presidential campaign I made clear there were profound differences between me and President Trump on a range of issues."