U.S. Republican presidential front-runner Donald Trump got a fresh injection of campaign momentum on Thursday with news that former rival Ben Carson, who is popular with conservatives, planned to endorse him.

Trump made it official at Thursday night's GOP debate.

He said he wanted Carson, a retired neurosurgeon who dropped out of the race March 4 after failing to gain traction in early voting states, to be heavily involved in education in his administration.

The endorsement could help Trump settle the nerves of those conservative voters who have doubts about whether he truly is one of them.

Carson's plans, first reported by The Washington Post, emerged just before the start of a CNN-hosted debate at the University of Miami that will feature the remaining four Republican contenders: Trump, U.S. Senator Ted Cruz of Texas, U.S. Senator Marco Rubio of Florida and Ohio Governor John Kasich.

Trump has drawn a lot of crossover support from blue-collar Democrats, harnessing working-class anxieties about immigrants and trade to supercharge his bid to become the Republican Party's candidate in the Nov. 8 election.

His White House bid has driven a wedge between the party's moneyed donors and grassroots voters, alarming party leaders who see him as a dangerous demagogue.

Trump's raucous rallies occasionally have descended into violence, including an attack on Wednesday night on a black protester in North Carolina.

Cruz, a first-term U.S. senator, is known for antagonizing fellow Republicans in Washington. On Thursday, he snared his first endorsement from a Senate colleague.

"Ted doesn't believe you have to settle," Senator Mike Lee of Utah told a Miami news conference.

Cruz, 45, aims to push Rubio, also a first-term U.S. senator, and Kasich off the campaign trail in order to position himself as the best Trump alternative.

"We think it's a two-man race," said Cruz backer Saul Anuzis, a former chairman of the Michigan Republican Party.