Ms Giulfoyle, a former Fox News host and the fiancée of Mr Trump’s eldest son Don Jr, dared the network to “try to stop” them from joining the event in Milwaukee on Wednesday night.
“I think, unfortunately, for a media company that I used to work for, you would expect better,” she told Newsmax on Tuesday night.
“But this is just really pathetic messaging. They’re just stepping out on themselves here. It’s backfiring spectacularly. It’s taking distraction away from the focus on the debate.”
Ms Guilfoyle insisted that she Don Jr “will be there” and will be streaming content on their Rumble account.
She added: “And I’d just like to see somebody try to stop Don Jr and me from going someplace.”
Her fiery comments came after Fox News punched back at Mr Trump, restricting his aides from participating in the traditional post-debate “spin room” after the former president refused to participate in the debate.
Fox, which is hosting the debate in Milwaukee, will limit post-debate spin room appearances to aides of those candidates who are actually participating in the debate, Axios first reported.
While he has decided to boycott the primary debate citing his lead in the polls, Mr Trump planned to be represented in Milwaukee by Don Jr and Ms Guilfoyle in addition to a number of other close allies who are also planning to attend.
Among those expected to attend in addition to Mr Trump Jr and Ms Guilfoyle are Ohio Senator JD Vance, former Arizona gubernatorial candidate Kari Lake, Georgia Rep Marjorie Taylor Greene, and Florida reps Matt Gaetz and Byron Donalds. It now appears they won’t be able to hype their candidate in the spin room.
The Messenger reported on Monday that there has been a move by campaign staffers of some other candidates to bar Trump representatives from monopolising the spin room at the Fox News-hosted event arranged by the Republican National Committee.
“If we didn’t agree to the RNC rules, I wouldn’t be let in the room, so why should they?” one staffer told The Messenger.
An adviser to one rival candidate called the possibility of Mr Trump not showing up as “bulls***, total lunacy”.
“The best part is the irony that the dude doesn’t have the stones to show up but he’s going to send Matt Gaetz,” they added.
A Republican official explained that it is up to each network who receives passes to the room, as it is the media organisations who have paid for the space at the venue.
Fox News’ host Bret Baier, who will be moderating the debate along with Martha MacCallum, clarified that Mr Trump’s surrogates can enter the spin room – but only by invitation from a media organisation.
“As far as in the spin room, it is the candidates on the stage and their delegates,” he said.
“And then if any media organisation who’s in the spin room invites one of these surrogates, a Trump surrogate, they are welcome to come. They have to get the invite from the media person in the room.”
While some staffers for candidates have been discussing the best way in which to lobby Fox News to stop Trump allies from swamping the airwaves, others have not engaged, seeing it as a losing issue.
Instead of appearing beside the other contenders for the party’s nomination, the former president will instead give an interview online to former Fox News presenter Tucker Carlson.
Over the weekend in a post on Truth Social, Mr Trump appeared to rule out participating in any of the debates scheduled for the primary season.
He wrote: “The public knows who I am & what a successful Presidency I had, with Energy Independence, Strong Borders & Military, Biggest EVER Tax & Regulation Cuts, No Inflation, Strongest Economy in History, & much more. I WILL THEREFORE NOT BE DOING THE DEBATES!”
Those candidates who have met the criteria to appear at the first debate are Florida Governor Ron DeSantis, tech entrepreneur Vivek Ramaswamy, former Vice President Mike Pence, Senator Tim Scott, former New Jersey Governor Chris Christie, former UN Ambassador Nikki Haley, North Dakota Governor Doug Burgum, and former Arkansas Governor Asa Hutchinson.
The second will take place at the Ronald Reagan Presidential Library in California on 27 September.