Republican presidential front-runner Donald Trump is once again waging a one-man war against Fox News host Megyn Kelly.

On Saturday, Trump tweeted that Kelly shouldn't be allowed to co-moderate the Fox News GOP primary debate that will take place Thursday in Des Moines, Iowa, because of a supposed "conflict of interest."

Trump did not specify what Kelly's "conflict of interest" was, but his tweet came two days after Kelly featured a number of conservative voices on her program who were involved in National Review's recent anti-Trump symposium, including Brent Bozell, Dana Loesch, Katie Pavlich and editor Rich Lowry.

The Trump team did not immediately respond to a request for comment.

Trump has already attacked Kelly several times during his campaign. During the first Republican primary debate in August, Kelly asked the candidate about a number of disparaging comments he'd made about women.

"You’ve called women you don’t like ‘fat pigs,’ ‘dogs,’ ‘slobs’ and ‘disgusting animals,’” Kelly said during the first debate, before Trump interjected, "Only Rosie O'Donnell."

In an interview with CNN's Don Lemon following the debate, Trump lobbed what many took to be a sexist insult at Kelly, saying: "You could see there was blood coming out of her eyes. Blood coming out of her -- wherever."

Katrina Pierson, Trump's campaign spokeswoman, attempted to downplay the candidate's tweet on Sunday, telling CNN's "Reliable Sources" that Trump isn't planning on skipping the Des Moines debate, despite Kelly's involvement.

"There’s no love lost between Megyn Kelly and Donald Trump after that first debate. That's been a very publicized disagreement there," Pierson told host Brian Stelter. "But I think it's going to be fine."

A Fox spokesperson confirmed to The Huffington Post that Kelly will indeed co-moderate Thursday's debate.

"Megyn Kelly has no conflict of interest," the spokesperson said. "Donald Trump is just trying to build up the audience for Thursday's debate, for which we thank him."