By Tim Reid

Former Vice President Mike Pence, who served Donald Trump with unwavering loyalty but later turned on him after the 2021 attack on the U.S. Capitol, will formally challenge his former boss for the Republican presidential nomination on Wednesday.

It is extremely rare for a vice president to run against a president he served under, and it has happened just a handful of times in U.S. history. Pence enters the Republican presidential primary with a mountain to climb, polling at just 5% and trailing Trump by 44 points, according to a Reuters/Ipsos opinion poll in May.

Pence, who turns 64 on Wednesday, will face Trump and at least 10 others in a crowded Republican field that is essentially a two-man race between Trump and Florida Governor Ron DeSantis.

Pence will launch his campaign at an event near Des Moines, Iowa's capital. He will follow that with a CNN town hall event on Wednesday evening. Pence's campaign declared his candidacy to the Federal Election Commission on Monday.

Pence, a conservative Christian, will focus much of his campaigning on Iowa, the first state to vote in the nominating contest next year. Iowa has a significant number of evangelical voters among its Republican electorate. Pence hopes a strong showing in the state will give him momentum and propel him into contention.

During Trump's tumultuous four years in the White House, Pence repeatedly defended him through multiple scandals.

But Pence incurred the wrath of Trump and his supporters when, as ceremonial president of the Senate, he refused to stop the certification of Democrat Joe Biden's victory over Trump in the 2020 election.

Pence said he had no constitutional authority to meddle with the election results. Trump supporters stormed the Capitol during the certification process on Jan. 6, 2021, forcing Pence, lawmakers and staff to flee to safety.

"I had no right to overturn the election, and his reckless words endangered my family and everyone at the Capitol that day, and I know that history will hold Donald Trump accountable," Pence said in March.

In Tweets during the certification, Trump accused Pence of cowardice. Some rioters chanted for Pence to be hanged.

Many of Trump's diehard supporters view Pence's refusal to overturn the election result as treachery, potentially complicating his path to the nomination.

Pence, who served as a governor of Indiana and is a former congressman, still embraces many of Trump's policies while portraying himself as an even-keeled and consensus-oriented alternative.

The success of his campaign will hinge on whether he can attract enough backers of Trump's policies who are turned off by the former president's rhetoric and behavior to build a viable coalition.

(Reporting by Tim Reid, editing by Ross Colvin and Grant McCool)