The two Republican front-runners had largely held their fire earlier in the race, but now they're feuding over Carson's childhood accounts. The war between the two has intensified, with Trump comparing Carson to a “child molester” and asking Thursday night "How stupid are the people of Iowa to believe this crap?"
It didn't always used to be like this, though.
At an event in South Carolina on Aug. 27, Trump complimented Carson, calling him a "really fine man" and "a friend of mine." Trump said the cordial relationship with Carson had led him to refrain from launching attacks on the campaign trail.
"I cannot hit him. He's been so nice to me," the businessman said.
But Trump did hit Carson a few weeks later, after the former neurosurgeon questioned Trump's faith. Fighting back, Trump complained that Carson was low energy and said that he was an "okay doctor."
Carson quickly decided he wasn't "going to participate" in the feud any longer, and apologized for his comments.
“The media frequently wants to goad people into wars, into gladiator fights, you know,” Carson told The Washington Post. “And I’m certainly not going to get into that.”
At the second GOP debate, everything seemed to have been smoothed over. The two even shared a high-five onstage.
In early October, Trump took to Twitter to defend Carson after the candidate faced criticism for saying he would have charged the gunman who killed ten at a mass shooting in Umpqua, Ore.
"Ben Carson was speaking in general terms as to what he would do if confronted with a gunman, and was not criticizing the victims," Trump said. "Not fair!"
But things quickly took a turn for the worse. On Oct. 24 Trump expressed skepticism about Carson's faith (Carson is a Seventh Day Adventist) contrasting it with his own "middle of the road" Presbyterian beliefs. Carson struck back at a rally during his book tour, saying he hoped Trump would apologize.
"Well you know a couple of months ago I said something which he took as an attack on his faith and I apologized for that," Carson said. "I hope he will have the same grace."
Now the two have come to blows over Carson's description of his childhood violence in his biography Gifted Hands
On Thursday, Trump mocked Carson’s claim that he attempted to stab a friend or relative and that Carson's knife blade broke before he could succeed.
"It hit the belt. And the knife broke. Give me a break," Trump said at an Iowa rally.
"He hit the belt buckle? Anybody with a knife want to try it on me? Believe me it ain't gonna work,” he said.
“It’s not the kind of dialogue that I would ever engage in,” Carson said of Trump’s comments this morning at a press conference. “I'm hopeful maybe his advisers will help him to understand the word pathological and recognize that that does not denote incurable.”
It seems that the closer Carson and Trump have come in the polls, the less friendly they have been to each other. But maybe the relationship between the two candidates hasn't always been so civil after all. In early September, Trump told "Good Morning America" he liked Carson "a lot," but quickly followed his compliments with a reminder that Carson was trailing in Iowa polls.
"He’s a good guy but he’s been spending a tremendous amount of advertising money in Iowa," Trump said.
ABC's Ryan Struyk, Katherine Faulders and John Santucci contributed to this report