"I'm all for party unity, but I'm not sure we have to pay for that with the secretary of state position," Kellyanne Conway, Trump's campaign manager, told CNN's Dana Bash on "State of the Union."
She added: "It's just breathtaking in scope and intensity the type of messages I've received from all over the country ... the number of people who feel betrayed to think that Gov. Romney would get the most prominent Cabinet post after he went so far out of his way to hurt Donald Trump."
Conway first opened a window on the infighting in Trump's transition over who should win the powerful role as the nation's top diplomat, when she tweeted on Thanksgiving about concerns with Romney
"Receiving deluge of social media & private comms re: Romney Some Trump loyalists warn against Romney as sec of state," she tweeted, including a link to a Politico story.
Romney has been scrapping behind the scenes with Rudy Giuliani for the post, although alternative options have emerged, including former Gen. David Petraeus and Tennessee Sen. Bob Corker.
Conway said Sunday that much of the anger, which has boiled over in public, stems from the personal attacks Romney lobbed at him.
"There was the Never Trump movement and then there was Mitt Romney he gave speeches against Donald Trump, he attacked his character," Conway said.
Ana Navarro, a Republican Trump critic and CNN political commentator, registered her surprise at the interview on Twitter, saying: "Astounding to hear K. Conway, who has the ability to tell Trump privately, trash possibility of Romney as Sec of State publicly on @CNNSotu."
But Conway fired back: "I did tell him privately. And I'll respect his decision. Point is the volume & intensity of grassroots resistance to Romney is breathtaking."
The fight between Romney, the Republican Party's 2012 nominee, and Trump was nasty and personal.
After Romney said he didn't expect Trump to win the GOP nomination and called some of the real estate mogul's past comments "childish in some respects," Trump tweeted: ".@MittRomney can only speak negatively about my presidential chances because I have been openly hard on his terrible "choke" loss to Obama!"
Trump fired back, saying that he could have forced Romney to his knees in 2012 when Romney sought his endorsement.
"He was begging for my endorsement. I could have said, 'Mitt, drop to your knees' -- he would have dropped to his knees," Trump said.
Romney's speech sparked a furious back-and-forth that only ended after the election when, in a stunning turn, the two met
to discuss Romney becoming secretary of state. Since that meeting, conservative backers of Trump have been openly bashing Romney, including former Arkansas Gov. Mike Huckabee and former House Speaker Newt Gingrich.
Reince Preibus, Trump's incoming chief of staff, downplayed any fighting behind the scenes, suggesting that picking Romney would make for a good "team of rivals" for Trump.
"He's going to be making the best decision for the American people. It isn't a matter of warfare. I mean, there's a lot of opinions about this and, yes, it is sort of a 'team of rivals' concept if you were to go towards the Gov. Romney concept," Priebus said on Fox News' "Fox News Sunday."
Priebus declined to say whether Romney should publicly apologize for his attacks on Trump. He also spoke highly of Petraeus, as a possible third option.
"I don't think anyone could say that David Petraeus isn't a very bright, calculated, smart person, and these are the types of conversations that I think the American people would expect of an incoming president that's trying to make the best decisions possible for everyone out there across the country," Priebus said.
Petraeus was sentenced to two years probation last year
after he revealed he had provided top secret information to his biographer, whom he was having an affair with. Trump often held out Petraeus on the campaign trail as evidence that Hillary Clinton should face prosecution for her use of a private email server.