(Reuters) - Former Vice President Mike Pence and former New Jersey Governor Chris Christie are planning to enter the race for the 2024 Republican presidential nomination next week in long-shot challenges to the dominance of front-runner Donald Trump.

Christie, who advised Trump's campaign in 2016 only to become a vocal critic of the former president, will formally announce his 2024 campaign on June 6, a person familiar with the matter said.

Pence, who incurred Trump's wrath by refusing to support his effort to overturn the results of the 2020 election, is set to enter the race against his former boss on June 7, three sources familiar with the situation told Reuters.

North Dakota Governor Doug Burgum is also planning to enter race on the same day as Pence, according to a person familiar with Burgum's plans.

The growing Republican field concerns many Trump opponents inside the Republican Party. They fear a large number of challengers will splinter the anti-Trump vote and hand the party's nomination to the former president, who can rely on at least 30% of the Republican base to back him.

Trump has a massive polling lead in a Republican field that now has more than 10 declared or almost-declared candidates. His nearest challenger is Florida Governor Ron DeSantis, who entered the race last week.

On Wednesday, Pence's polling average in the Republican field was less than 4%, compared to Trump's 53%, according to a RealClearPolitics average of polls. Christie to date has barely registered in the polls.

Pence will launch his campaign with a video and a speech in the early nominating state of Iowa, the sources said.

A staunch social conservative who stood by Trump throughout his time in his office, Pence has increasingly distanced himself from the former Republican president since his election defeat, saying Trump's encouragement of the rioters who attacked the U.S. Capitol on Jan. 6, 2021, put him and his family in danger.

Pence, who turns 64 on June 7, has continued to embrace many of Trump's policies, while portraying himself as an even-keeled and consensus-oriented alternative. He has also appealed more directly to the evangelical Christian community.

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.