U.S. President Donald Trump took a swipe Monday while at a rally in Pennsylvania at Fox News, the conservative news network credited with helping propel his winning 2016 presidential campaign.

"What's going on with FOX? What's going on there? They're putting more Democrats on than you have Republicans," said Trump.

"Something strange is going on at FOX! Something very strange. Did you see this guy last night? I did want to watch, you've always got to watch the competition if you call it that. And he was knocking the hell out of FOX and FOX is putting him on. Somebody is going to have to have to explain the whole FOX deal to me," Trump concluded.

Trump was referring to Mayor Pete Buttigieg's Sunday night town hall on Fox News, in which Buttigieg slammed Fox News' Tucker Carlson and Laura Ingraham and received a standing ovation from the audience at the end of the event.

Brit Hume, Fox's senior political analyst hit back at Trump on Twitter. Hume wrote, "Say this for Buttigieg. He’s willing to be questioned by Chris Wallace, something you’ve barely done since you’ve been president. Oh, and covering candidates of both parties is part of the job of a news channel."

Trump holds a 'MAGA' rally in Pennsylvania

Trump criticized Fox News before Sunday's event for “wasting airtime” on Buttigieg, saying Fox “is moving more and more to the losing (wrong) side in covering the Dems.” He added, “Alfred E. Newman will never be President.” Hume, later on-air praised Buttigieg, saying the mayor is the "most impressive, by far, candidate, in terms of just raw political talent in the Dem field." Hasn't seen anyone like it "since the emergence of Barack Obama."

Trump also took aim at Fox News for hosting a town hall with Bernie Sanders. "So weird to watch Crazy Bernie on @FoxNews," Trump tweeted after the event last month - also suggesting the audience which gave Sanders a warm reception were plants, by writing that the so-called "audience" was "so smiley and nice."

Trump voiced confidence in his ability to win a repeat victory in 2020 and took a fresh swipe at one of his leading Democratic rivals, telling rally goers that native son Joe Biden had abandoned them by representing Delaware in the Senate.

Trump, however focused his criticism on Biden, who actually moved to neighboring Delaware with his family when he was a boy, and later represented the state in the Senate for more than three decades. He maintained ties to Pennsylvania over the years.

Trump's Pennsylvania visit, intended to boost Republican congressional candidate Fred Keller over Democrat Marc Friedenberg in a Tuesday special election for an open House seat, had as much to do with helping his own reelection prospects as it did with pushing Keller over the finish line.

"We've got to win tomorrow, Fred," Trump told a cheering rally crowd at a hangar at Williamsport Regional Airport.

Trump's visit to the key battleground state came two days after Biden held a campaign rally in Philadelphia, and the former vice president wasn't far from Trump's mind.

"He left you for another state, and he didn't take care of you," Trump said. He also referred to the former vice president by the nickname he has coined for him: "Sleepy Joe."

"Sleepy Joe said that he's running to, quote, 'save the world,'" Trump said. "Well, he was. He was going to save every country but ours."

Biden said Monday in Nashville, Tennessee, that he is running on a pledge to restore the soul of America. He has frequently talked on the campaign trail about the president's divisive rhetoric and said another four years of Trump would "fundamentally change the character of this nation."

Trump uses his campaign rallies to disparage various Democratic candidates for president, but he has been heavily focused on Biden, suggesting he may be worried about the possibility of facing off next year against the longtime politician.

The president, who spoke in the open air with Air Force One behind him, highlighted the economy's performance under his leadership and suggested those numbers make him virtually unbeatable.

"Politics is a crazy world, but when you have the best employment numbers in history, when you have the best unemployment numbers in history ... I don't know, how the hell do you lose this election, right?" Trump said. The current unemployment rate of 3.6% is actually the lowest since 1969, when it stood at 3.5%. Unemployment was even lower than that in the early 1950s, and much lower, under 2%, during three years of World War II.

Keller himself offered a rousing endorsement of Trump, saying he wants to go to Congress to be a vote for the president. Keller told Trump the people of this region of Pennsylvania "have been behind you since Day One, and, Mr. President, our support for you is as strong today as it ever was."

"In 2016, Pennsylvania put Donald Trump over the top. And in 2020, we're going to do it again," Keller said.

Biden is making a big play for his native Pennsylvania, opening his presidential bid in Harrisburg and capping a three-week rollout with Saturday's event in Philadelphia, the city where he also established his campaign headquarters.

In the fight to deny Trump reelection, no places will matter more than Pennsylvania, Michigan and Wisconsin, three states the Republican president carried by razor-thin margins in 2016. Trump campaigned in Michigan and Wisconsin earlier this year.