Ben Carson, who dropped out of the Republican presidential race just last week, is expected to endorse front-runner Donald Trump on Friday, sources told Fox News.

The retired neurosurgeon plans to hold a press conference at 9 a.m. ET Friday to announce his support. The expected backing from the outsider ex-candidate, who captivated conservative voters with his unconventional campaign, could give Trump a boost as the candidates charge into critical primaries next Tuesday in Florida, Ohio and three other states. 

It also serves as a potential counterweight as other ex-candidates weigh in against him. Former HP CEO Carly Fiorina endorsed Ted Cruz earlier this week, and former Florida Gov. Jeb Bush is meeting with Trump’s rivals, though it’s unclear whether he might endorse someone before Tuesday. 

The Washington Post first reported on Carson’s expected endorsement. Sources told the Post that Carson agreed to the endorsement after a meeting with Trump at the billionaire's Mar-a-Lago luxury home in Palm Beach, Fla.

As Trump builds his delegate lead with a string of recent primary wins, the GOP candidates also were facing off at a debate Thursday night in Miami. 

Earlier on Thursday, Carson said he was "certainly leaning" toward endorsing Trump over Texas Sen. Cruz, during an interview with Fox News Radio's John Gibson.

"There are two Donald Trumps," Carson told Gibson. "There's the Donald Trump that you see on television and who gets out in front of big audiences, and there's the Donald Trump behind the scenes. They're not the same person." 

He said the other Donald Trump is “actually a thinking individual” and “someone you can reason with very easily.”