Former rival Ben Carson backed Republican presidential front-runner Donald Trump on Friday, becoming the second former presidential candidate to support the billionaire businessman in the race for the White House.

Carson, a retired neurosurgeon who dropped out of the race on March 4, follows New Jersey Governor Chris Christie, a former candidate who also backed Trump after earlier dropping out of the race for the party's nomination ahead of the Nov. 8 presidential election.

Carson made the announcement at a joint news conference with Trump in Florida, ahead of that state's primary on Tuesday.

Trump welcomed Carson's support, saying the doctor will be involved in education and in health-care policy in the Trump campaign.

"It's such an honour to have Ben. He's a friend and he's become a friend."

'2 Donald Trumps'

Carson echoed the admiration. "Donald Trump talks about making America great, but it's not just talk — he means it," Carson said.

Carson described "two Donald Trumps" — the persona reflected on stage, and a private, "very cerebral" person who "considers things carefully."

Trump, answering a reporter's question, agreed with the characterization: "I'm a deep thinker."

He went on to reiterate his confidence that a Trump administration would restore the United States to a position of economic greatness.

Regarding a violent episode at one of his rallies this week, Trump said the "audience swung back" at a white man who was caught on video hitting a black man as he was escorted out of the rally by deputies.

Trump praised the police as "amazing," saying they were "very restrained" in response to the incident.

'Presidents can't just say anything they want, because it has consequences around the world.' - Marco Rubio

He said that the man — identified as John Franklin McGraw — began hitting people, and the audience hit back. "That's what we need a little bit more of," he said.

Thursday night, a newly civil Republican debate still came with a warning from front-runner Trump to a party that's been at war with itself over his insurgent candidacy: "Be smart and unify."

While the debate focused on issues rather than insults, it wasn't clear that Ted Cruz, Marco Rubio and John Kasich were able to gain ground on the New York billionaire heading into the all-important primary votes Tuesday in Florida and Ohio.

Rubio, the Florida senator, must win his home state. The same holds for Kasich, the Ohio governor.

In all, 367 Republican delegates are at stake in Tuesday's voting that also takes place in Illinois, Missouri, North Carolina, and the Northern Mariana Islands, which could go a long way toward determining the Republican nominee.

Democrats Hillary Clinton and Bernie Sanders also will be competing on Tuesday, with Clinton out to regain momentum after her startling loss to Sanders in Michigan this week.

Consequences

The latest Republican debate — which was so calm that Trump called it "elegant" — still had its share of criticism.

Rubio's message: "I know that a lot of people find appeal in the things Donald says. The problem is, presidents can't just say anything they want, because it has consequences around the world."

Cruz, eager to cement his position as the party's last best alternative to Trump, said: "His solutions don't work."

Trump was clearly intent on projecting a less bombastic, and more presidential, image.

"We're all in this together," he said. "We're going to come up with solutions. We're going to find the answer to things."

In a discussion of the threat posed by radicalized Muslims, Trump refused to back away from his recent statement that "Islam hates the West." He said he wouldn't stoop to being "politically correct" by avoiding such statements.

Work with Muslims

Rubio had a sharp comeback: "I'm not interested in being politically correct. I'm interested in being correct."

The Florida senator said the only way to solve the problem of violent extremists is to work with people in the Muslim faith who are not radicals.

Cruz criticized Trump for what he called simplistic solutions on both trade and on Islamic terrorism, saying: "The answer is not to simply yell, 'China: bad, Muslim: bad."'

The candidates split on the likelihood of the Republican race coming down to a contested party convention this summer if no candidate has the needed majority of delegates.

In the race for Republican delegates, Trump has 459, Cruz 360, Rubio 152 and Kasich 54. It takes 1,237 to win the Republican nomination for president.

With files from The Associated Press