Miles Taylor, former Department of Homeland Security (DHS) chief of staff under Kirstjen Nielsen, became one of the highest-ranking former administration officials to endorse the president’s rival on Monday, featuring in a scathing attack ad.
Mr Trump hit back against the comments following the first night of the Democratic National Convention, calling Mr Taylor a “disgruntled employee” and insisting that he had never heard of him.
“Many thousands of people work for our government,” the president wrote on Twitter.
“With that said, a former DISGRUNTLED EMPLOYEE named Miles Taylor, who I do not know (never heard of him), said he left & is on the open arms Fake News circuit. Said to be a real ‘stiff’. They will take anyone against us!”
Mr Trump’s outburst follows Mr Taylor’s appearance in a video for Republican Voters Against Trump and his op-ed in The Washington Post on Monday.
“What we saw week in and week out, and for me, after two and a half years in that administration, was terrifying,” Mr Taylor, a longtime Republican, said in the video.
“We would go in to try to talk to him about a pressing national security issue – cyberattack, terrorism threat – he wasn’t interested in those things. To him, they weren’t priorities.”
Mr Taylor claimed that the president wanted to “restart” the practice of separating children from their families at the US-Mexico border following the termination of the widely-denounced strategy in 2018, among a number of allegations.
He added: “Given what I have experienced in the administration, I have to support Mr Biden for president, and even though I am not a Democrat, even though I disagree on key issues, I’m confident that Mr Biden will protect the country and I’m confident that he won’t make the same mistakes as this president.”
Mr Taylor’s wrote in his op-ed that the president is “governed by whim, political calculation and self-interest”.
According to Mr Taylor, the president allegedly wanted to “cut off the money and no longer give individual assistance” to people who suffered during deadly wildfires in California, as he believed the people in the state didn’t support him politically.
The former DHS official is among several high-profile former administration officials who have publicly denounced the president.
Anthony Scaramucci, who briefly served as White House communications director, has endorsed the Democratic nominee.
Former national security chief John Bolton, has said he will not be voting for Mr Trump or Mr Biden.