Donald Trump's fight with Fox News has exposed another personal feud -- the one between Fox stars Bill O'Reilly and Megyn Kelly.
The animosity between O'Reilly and Kelly is an open secret in the halls of Fox. People there disagree about its origins, but they agree the two prime time hosts are highly competitive with one another.
"He's never had a serious challenger like her before," said one high-ranking source at Fox.
The channel's ratings and brand haven't suffered as a result. But the tension between the two stars is a source of intrigue and sometimes a cause of headaches for Fox News chief Roger Ailes. Both hosts' contracts will come up for renewal next year.
The strained relationship was most recently visible on the air when O'Reilly interviewed Trump on Wednesday night. Trump had publicly pledged to skip Fox's GOP debate the next day -- a protest, he said, of Kelly's unfairness and Fox's defense of her. O'Reilly, who has been friends with Trump for decades, pleaded with him to reconsider.
The conversation stretched out over two blocks of O'Reilly's 8 p.m. show, so O'Reilly had plenty of time to stick up for his colleague, whom Trump had harshly criticized incessantly for nearly six months. But when Trump said he had "zero respect" for the "highly overrated" Kelly, O'Reilly was silent.
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?"
Related: Fox News says Donald Trump is scared of Megyn Kelly
Asked for comment for this story, a Fox News spokesperson said, "We're thrilled to have the two biggest stars in cable news on one network."
The animosity between O'Reilly and Kelly -- what one source called a "serious rivalry" between them -- seems to come from the all-important ratings race.
The two hosts, by all accounts, had a collegial relationship for years. When Kelly was an afternoon news anchor, O'Reilly (encouraged by Ailes) raised her profile with weekly "Kelly File" segments on "The O'Reilly Factor." In October 2013, when she took over the 9 p.m. hour, the show was named "The Kelly File."
But the relationship went downhill after Kelly moved in next door.
Here's the thing: O'Reilly has by far the biggest audience of anyone on cable news. He remains the undisputed "king," as one of his allies put it, with a 16-year winning streak among total viewers that fuels Fox's day-in-day-out success. So any show that follows him on Fox has the benefit of his lead-in.
Related: Bill O'Reilly blows up at George Will: 'You're a hack'
Kelly has been able to hold onto more of O'Reilly's viewers than did the previous 9 p.m. host, Sean Hannity. And she has attracted new viewers.
There is one demographic group cable news bosses care most about: viewers between the ages 25 and 54. O'Reilly continues to do better than Kelly in that demo most of the time.
But Kelly has recently scored some prominent wins.
In November 2014, Kelly beat O'Reilly for the whole month. But the win came with an asterisk: Rioting broke out in Ferguson, Missouri, during her hour, which lifted her viewership, and the big ratings from that one night were enough to notch a win for the whole month.
She beat him again in June 2015, when Kelly's win was propelled by an exclusive interview with the Duggar sisters.
Then came Fox's GOP debate on August 6.
Kelly was a co-moderator, drawing the wrath of Donald Trump for her questioning, and the debate drew 24 million viewers. Her post-debate "Kelly File" had upwards of 10 million. That was enough to put her ahead of "The O'Reilly Factor" for the whole month and quarter.
According to a well-placed source, O'Reilly argued to Ailes that Kelly's post-debate show should be labeled (for ratings purposes) a special, not a regular edition of her show.
But it counted. And Kelly won again in November, the month of the attacks in Paris.
Related from September: Kelly beats O'Reilly's decade-long streak
On one level Ailes encourages this kind of competitiveness. He is proud of Kelly's meteoric rise -- and yet he knows O'Reilly is the "flagship," as O'Reilly called himself last week, of Fox's prime time schedule. How high would her ratings be without his?
O'Reilly and Kelly rarely see each other at Fox News HQ. He usually tapes his hour in the afternoons while she is busy preparing for her live hour. So the spat mostly plays itself out through intermediaries.
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.
The two hosts compete for big bookings, too.
O'Reilly famously tangled with liberal filmmaker Michael Moore many years ago. But it was Kelly, not O'Reilly, who nabbed an interview with Moore last week. Why? Moore told CNN that he only wanted to appear on Fox live, not in a pre-taped setting that could be edited, and Kelly's show is live while O'Reilly's show is usually taped.
The rivalry, while well known in cable news circles, rarely spills into public view. And there are limits to the hostilities.
An example came up on Monday, when CBS announced that Kelly is booked on Stephen Colbert's post-Super Bowl show, a coveted spot because tens of millions of people are likely to tune in.
O'Reilly is booked on Colbert's show the next day to promote a new book and Fox's New Hampshire primary coverage. The prospect of a smaller audience -- being upstaged by Kelly -- could have turned off O'Reilly, but both appearances are going forward.
In the press, much has been made of the fact that Kelly's contract will expire sometime in 2017. Vanity Fair's recent cover story about Kelly ended with her saying she wanted "longer, more in-depth interviews" in her future, at Fox or at another network.
What hasn't been noted as often is that O'Reilly's contract expires sometime after the presidential election, too.
"Of course Fox News wants to keep Megyn happy. I would argue that she is the brightest star on television right now," former Fox anchor Laurie Dhue said on Sunday's "Reliable Sources." But Dhue quickly added: "Bill O'Reilly helped make her a star."
