Donald Trump turned his fire on Fox News, threatened to have his attorney general investigate Democrats and FBI for treason and joked about serving five terms as president during a typically rambling rally speech in Pennsylvania.
The president was in the northeastern state to campaign for Republican congressional candidate Fred Keller, ahead of a special election on Tuesday.
“What’s going on with Fox, by the way?” Mr Trump asked the crowd.
“They’re putting more Democrats on than Republicans. Something strange is going on at Fox, folks! Something very strange!”
His remarks caused the crowd to loudly boo the right-wing television network, which has previously shown staunch support to the president.
Mr Trump had earlier accused Fox News of “moving...to the losing (wrong) side” in a series of tweets posted on Sunday.
The president appeared angered by the cable network’s decision to screen a televised town hall event featuring Pete Buttigieg.
“Hard to believe that Fox News is wasting airtime on Mayor Pete,” the president wrote.
“Fox is moving more and more to the losing (wrong) side in covering the [Democrats].
“They got dumped from the Democrats boring debates, and they just want in.
“They forgot the people...who got them there.”
The president has expressed increasing frustration with Fox News over recent weeks, despite the network’s consistently pro-Trump stance.
During Monday’s rally the president also criticised Democratic frontrunner Joe Biden.
“I guess he was born here, but he left you folks,” Mr Trump claimed.
“He left you for another state. Remember that please. I meant to say that.”
The president also accused the FBI and Democrats of “treason”over the investigation of his 2016 election campaign for its links to Russia, as his supporters chanted "lock them up".
“Well we have a great new attorney general, who is going to give it a very fair look,” he said.
The 72-year-old also discussed serving four more years after 2020 and joked about remaining in office for up to five full terms.
“We’ll go and we’ll do what we have to do. We’ll do a three and a four and a five [terms],” he said.
Additional reporting by agencies