Ms McEnany, 35, served as the former president's fourth press secretary and was touted to be one of his most loyal aides before joining Fox News as a contributor.
"Kayleigh 'Milktoast' McEnany just gave out the wrong poll numbers on Fox News," Mr Trump wrote on Truth Social, while incorrectly spelling the term milquetoast.
"I am 34 points up on DeSanctimonious, not 25 up. While 25 is great, it’s not 34," the one-time president said, using his nickname for Florida governor Ron DeSantis.
"She knew the number was corrected upwards by the group that did the poll. The RINOS & Globalists can have her. FoxNews should only use REAL Stars."
During her appearance on Fox New's “Outnumbered”, Ms McEnany discussed recent polling in Iowa which suggested Mr DeSantis was “narrowing the gap” with the former president since he announced his run for the president's office.
It was not clear which poll Mr Trump and Ms McEnany were referencing, although a poll from Emerson College Polling showed the former president leading Mr DeSantis by 42 points in Iowa.
Mr Trump’s "erratic" rant on Truth Social left the Republicans livid who said they were upset and "at a loss for words".
"Kayleigh is the kindest person and was the best press secretary to ever step foot in the White House," tweeted user Kambree. "There is ZERO excuse for degrading or belittling her to this degree over a few points."
"Praising Andrew Cuomo. Insulting Kayleigh McEnany .… what happened to Trump," asked Christina Pushaw.
"He’s completely unhinged," said columnist Marc Thiessen on Twitter.
"Kayleigh McEnany was one of the best people to serve in his administration. He’s losing control, lashing out at anyone and everyone who does not tow his line. Sad."
“TRASH. Kayleigh was one of THE greatest talents in the trump admin. He knows it, we know it, and conservative leaning suburban female voters know it,” tweeted Alex Clark.
“This is perhaps Trump’s lowest moment. Kayleigh McEnany defended Trump during some of the most trying moments of his presidency while enduring endless personal attacks from the media,” wrote Bradley Stein.
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