Sean Hannity appeared at the podium in a Missouri arena after being called up to the stage by Trump. He attacked members of the media covering the rally, saying, "By the way, all those people in the back are fake news".
"The Trump agenda is paying massive dividends", Hannity said.
Observers accused Hannity, Fox News' biggest star, of campaigning for the president as he ostensibly represented a network of journalists.
Trump has punched back, accusing Obama of leaving behind a trail of broken promises on trade, the economic recovery and a promise during his presidency that patients could keep their doctors under his health care law.
As race calls stream in from the Fox News Decision Desk, viewers will see live updates on the projections for the balance of power in Congress.
That has not been the case for Hannity however, who previously suffered no fallout when it was revealed he was a client of lawyer Michael Cohen, who at the time was also President Trump's legal advisor.
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Some of Fox News' biggest names, including Hannity, the hosts of "Fox & Friends", and Jeanine Pirro - who also appeared with Trump on stage Monday night - have turned into full-time Trump supporters on television over the past two years.
Already No. 1 in the cable news ratings, Fox has had a strong fall with the campaign and hearings for Supreme Court Justice Brett Kavanaugh, with Hannity leading the way.
"It disturbs me to my core", another senior Fox staffer said. A Fox spokeswoman said she didn't know how that impression had been created.
"The campaign started with talk of [the] blue wave", said Fox News' chief national correspondent Ed Henry on "The Ingraham Angle" on Monday night.
"We have an extraordinary team of journalists helming our coverage tonight and we are extremely proud of their work", the statement added.
Earlier yesterday, Hannity tweeted, "To be clear, I will not be on stage campaigning with the President".