Washington (CNN) President-elect Donald Trump is looking to some of his most bitter past opponents and critics as he assembles a Cabinet.

Former Massachusetts Gov. Mitt Romney went from being "one of the dumbest and worst candidates in the history of Republican politics," who "choked like a dog" in his 2012 run against President Barack Obama, according to Trump, to now being considered for secretary of state.

Mitt Romney had his chance to beat a failed president but he choked like a dog. Now he calls me racist-but I am least racist person there is

South Carolina Gov. Nikki Haley went from being "very weak on immigration" and a governor who has "embarrassed" her state to being selected by Trump Wednesday as US ambassador to the UN.

And Dr. Ben Carson went from being a GOP primary rival who is "incapable of understanding foreign policy," and someone who "has never created a job in his life (well, maybe a nurse)," to "a greatly talented person," whom Trump is "seriously considering" as the next Housing and Urban Development secretary.

Ben Carson has never created a job in his life (well, maybe a nurse). I have created tens of thousands of jobs, it's what I do.

Here's a look at some of Trump's former rivals, many of whom he engaged in fierce feuds with during the 2016 primary, who have now been selected or could be chosen to fill top positions in a Trump administration.

Mitt Romney

News that Romney could join a Trump administration, much less to fill a top job like secretary of state, came as a surprise to many.

One of the memorable moments in the 2016 campaign and one that crystallized the bitter feud between establishment and anti-establishment Republicans was Romney's brutal take-down of Trump.

The 2012 Republican presidential nominee assailed Trump before an audience at the University of Utah in March, slamming the billionaire mogul as "a phony" and a "fraud."

"His promises are as worthless as a degree from Trump University," Romney said. "He's playing members of the American public for suckers: He gets a free ride to the White House, and all we get is a lousy hat."

Trump quickly returned fire.

"Looks like two-time failed candidate Mitt Romney is going to be telling Republicans how to get elected. Not a good messenger!" Trump tweeted.

Looks like two-time failed candidate Mitt Romney is going to be telling Republicans how to get elected. Not a good messenger! — Donald J. Trump (@realDonaldTrump) March 3, 2016

Mitt Romney, who was one of the dumbest and worst candidates in the history of Republican politics, is now pushing me on tax returns. Dope! — Donald J. Trump (@realDonaldTrump) February 25, 2016

"@MittRomney was a disaster candidate who had no guts and choked! Romney is a total joke, and everyone knows it!" Trump added.

.@MittRomney was a disaster candidate who had no guts and choked! Romney is a total joke, and everyone knows it! — Donald J. Trump (@realDonaldTrump) March 5, 2016

In June, Romney told CNN's Wolf Blitzer that he would not back Trump and said a Trump presidency could lead to "trickle-down racism."

Trump tweeted, "Mitt Romney had his chance to beat a failed president but he choked like a dog. Now he calls me racist-but I am least racist person there is."

Failed candidate Mitt Romney,who ran one of the worst races in presidential history,is working with the establishment to bury a big "R" win! — Donald J. Trump (@realDonaldTrump) March 3, 2016

Trump, who had endorsed Romney in 2012, consistently slammed the former Massachusetts governor following Romney's loss to Obama in 2012.

"If Mitt Romney were in the private sector & he suffered the horrendous loss of 2012, do you think he'd rehire himself for 2016?—I don't!" Trump tweeted in 2014.

If Mitt Romney were in the private sector & he suffered the horrendous loss of 2012, do you think he'd rehire himself for 2016?—I don't! — Donald J. Trump (@realDonaldTrump) October 22, 2014

Nikki Haley

News that Haley accepted Trump's offer to become the next US ambassador to the UN is another unexpected twist.

The South Carolina governor, who initially endorsed Florida Sen. Marco Rubio, sharply disavowed Trump and later backed his rival, Texas Sen. Ted Cruz, after Rubio dropped out of the race.

The people of South Carolina are embarrassed by Nikki Haley! — Donald J. Trump (@realDonaldTrump) March 1, 2016

Trump said in January that Haley is "very weak on illegal immigration" and tweeted in March, "The people of South Carolina are embarrassed by Nikki Haley!"

Haley did not back down and said in February that Trump represents "everything a governor doesn't want in a president."

Ben Carson

Early on in the primary contest, Carson briefly began to lead Trump in some national polls, turning what had begun as a friendly relationship into to a rivalry defined by personal attacks and insults, largely from Trump.

Trump slammed Carson for stories that the famed neurosurgeon tells in his book about his childhood, and accused his opponent of lying about his past.

"The Carson story is either a total fabrication or, if true, even worse-trying to hit mother over the head with a hammer or stabbing friend!" Trump tweeted.

The Carson story is either a total fabrication or, if true, even worse-trying to hit mother over the head with a hammer or stabbing friend! — Donald J. Trump (@realDonaldTrump) November 6, 2015

"With Ben Carson wanting to hit his mother on head with a hammer, stab a friend and Pyramids built for grain storage - don't people get it?" he added.

With Ben Carson wanting to hit his mother on head with a hammer, stab a friend and Pyramids built for grain storage - don't people get it? — Donald J. Trump (@realDonaldTrump) November 6, 2015

Trump also told CNN's "New Day" that Carson -- the first surgeon to separate twins joined at the head -- is only an "OK doctor" and said , "You look at his faith and I think you're not going to find so much."

Carson attempted to avoid personal attacks and later endorsed Trump in March, a week after ending his own campaign.

Bobby Jindal

The Louisiana governor, who is rumored to be considered for the position of Secretary of the Health and Human Services, once called Trump an "egomaniacal madman" and a "non-serious carnival act."

Trump shot back against his then-Republican primary opponent, calling Jindal "a little bit nasty," but adding that he is ultimately "a nice guy."

Jindal later endorsed Trump.

Rick Perry

Former Texas Gov. Rick Perry is said to be under consideration as Trump's Secretary of Energy pick. Perry was one of Trump's first and most outspoken opponents during the primary race and once called Trump "a cancer on conservatism."

Trump repeatedly hit the former Texas governor on immigration and questioned his intelligence.

"Rick Perry failed at the border. Now he is critical of me. He needs a new pair of glasses to see the crimes committed by illegal immigrants," Trump tweeted.

Rick Perry failed at the border. Now he is critical of me. He needs a new pair of glasses to see the crimes committed by illegal immigrants. — Donald J. Trump (@realDonaldTrump) July 5, 2015

.@GovernorPerry failed on the border. He should be forced to take an IQ test before being allowed to enter the GOP debate," Trump followed up in June.

.@GovernorPerry failed on the border. He should be forced to take an IQ test before being allowed to enter the GOP debate. — Donald J. Trump (@realDonaldTrump) July 16, 2015

"Rick Perry did an absolutely horrible job of securing the border. He should be ashamed of himself. Gov. Abbott has since been terrific," he added a few days later.

Rick Perry did an absolutely horrible job of securing the border. He should be ashamed of himself. Gov. Abbott has since been terrific. — Donald J. Trump (@realDonaldTrump) July 21, 2015

But Perry backed Trump in May after Cruz quit the race.

Ben Bernanke

The former chairman of the Federal Reserve is reportedly being eyed for the Secretary of the Treasury position, although Trump has slammed Bernanke in the past.

"With Obama and Bernanke destroying the value of the dollar, gold and real estate should continue to rise in value," he tweeted in 2012.