President Donald Trump on Monday continued to prosecute his grudge against Fox News for giving a platform to Pete Buttigieg, a 2020 Democratic presidential candidate.

At a rally in Pennsylvania, Trump said: "Something strange going on at Fox, folks. Something very strange!" The crowd booed in response.

Trump was unhappy with Fox for broadcasting Buttigieg speaking with voters in Claremont, New Hampshire, on Sunday.

Just before the event on Sunday, Trump tweeted that Fox was "wasting airtime" and should ignore Democrats to cover him more.

Buttigieg was asked about Trump's tweets on air and said, "The tweets are — I don't care."

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At a rally in Montoursville, Pennsylvania, on Monday night, President Donald Trump made it clear that he's still holding a grudge against Fox News for giving airtime to Pete Buttigieg, a 2020 Democratic presidential candidate.

Trump made several jabs at his favorite news network, prompting a chorus of boos at Fox's expense from the large crowd.

Fox on Sunday broadcast Buttigieg speaking with voters in Claremont, New Hampshire. In tweets on Sunday, Trump said Fox was "wasting airtime" by showing the event.

Trump told supporters in Montoursville that "something strange" was affecting Fox. He mentioned Buttigieg, again comparing him to the Mad magazine cartoon character Alfred E. Neuman.

"Last night I watched Alfred E. Neuman. What's going on with Fox, by the way? What's going on there?" Trump said.

"They're putting more Democrats on than you have Republicans. Something strange going on at Fox, folks. Something very strange!"

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Trump continued: "Did you see this guy last night? I did want to watch — you always have to watch the competition, if you call it that.

Trump boarding Air Force One after the rally. Reuters

"He was knocking the hell out of Fox," Trump said, adding, "Somebody's going to have to explain the whole Fox deal to me."

Fox also broadcast Trump's rally, including his criticism of the network.

Pete Buttigieg at a town hall in Claremont, New Hampshire, on Sunday. Fox News

Buttigieg had used part of his town hall in Claremont to criticize the Fox hosts Tucker Carlson and Laura Ingraham for their coverage of the migrant crisis at the US-Mexico border. (Neither was at the event.)

He chastised Carlson for "saying immigrants make America dirty" and Ingraham for "comparing detention centers with children in cages to summer camps."

Read more: 'Boot-EDGE-EDGE': Trump tries to pronounce 2020 Democratic candidate Pete Buttigieg's name at Florida rally

During the broadcast, the Fox host Chris Wallace asked Buttigieg about Trump's Twitter outburst.

Buttigieg speaking to Chris Wallace on Fox News on Sunday. Fox News

"The tweets are — I don't care," Buttigieg replied.

Buttigieg said it's "hard for anyone to look away" when Trump starts tweeting, adding, "It is the nature of grotesque things that you can't look away."

Buttigieg got a standing ovation from the New Hampshire crowd.