Megyn Kelly and Bill O’Reilly are two of the highest rated and highest paid personalities on cable news, and the undisputed stars of the Fox network.

Perhaps unsurprisingly, in the halls of Fox News, it’s well known that that there’s a rivalry between the two Catholic, Irish American political pundits.

But as Brian Stelter reported this week for CNN, the feud between them is growing in light of their almost neck-and-neck ratings race and what many see as O’Reilly’s betrayal of his colleague in not challenging Donald Trump in his campaign against Kelly acting as a moderator of the last Republican primary debate before the Iowa caucuses.

After Kelly questioned Trump during the first Republican debate about his history of sexist remarks against women, the Republican presidential candidate began a public campaign against her, implying on CNN that she had been menstruating when she asked the question, calling her a “bimbo” on Twitter, and demanding that Fox remove her as moderator if they wanted him to participate in the most recent debate.

Fox declined, suggesting that Trump should grow a thicker skin if he wants to be president. Trump ignored Fox's suggestion and carried through on his threat to boycott the debate. However, he did appear on Fox the night before, on the "O’Reilly Factor." And when he reiterated his decision to skip the debate, calling Kelly “highly overrated” and declaring that he has “zero respect” for her, O’Reilly declined to defend his colleague in any way.

As Stelter wrote:

“Kelly's own show started 45 minutes later. She didn't say a word about it. But inside Fox, others were buzzing. Ailes was not happy. One Kelly supporter went so far as to call it a ‘betrayal.’

“Outside Fox, the snub was widely noticed. O'Reilly ‘let all the attacks stand,’ MSNBC's Joe Scarborough said the next morning.

“Scarborough's co-host Mika Brzezinski added: ‘O'Reilly should have said 'Excuse me? Excuse me? You're talking about her intellect? She's a lawyer. She's done this. She's done that. She's a mother of three. You do not, you do not undermine the credibility and the intellect of my colleague.' Where was that, Bill? Hello?’"

Stelter asked Fox News for a comment, but received the rather vanilla, "We're thrilled to have the two biggest stars in cable news on one network,” in reply.

But he does quote inside sources at Fox who claim the network is very aware of the “serious rivalry” between Kelly and O’Reilly, and that "he's never had a serious challenger like her before.”

"The Kelly File" was initially a segment on "The O'Reilly Factor," before Kelly for got her own live show by the same name for the 9pm slot in 2013. At the time, Kelly made it very clear that she was "not going to be the female Bill O'Reilly.

While "The O’Reilly Factor" has overall maintained slightly higher ratings than "The Kelly File," Kelly did beat O’Reilly in ratings for three months in 2015, leading to speculation that O’Reilly is feeling jealous and/or threatened.

As Stelter put it: “From Kelly's perspective, she has been nothing but kind to O'Reilly, while she thinks he lets jealousy get the best of him. From O'Reilly's perspective, he helped make Kelly a star but now she's too eager to outshine him – touting her own ratings successes without respecting his contributions.”

With both of their contracts with Fox up for renewal in the next year or so and a busy election season ahead, interesting developments are surely in store.