The Fox News host Bret Baier pushed back on US President Donald Trump's claim that the network had "changed" after it published a poll that found the president had less national support than four Democratic contenders.

Trump said on Sunday that "there's something going on at Fox" and that his "worst polls have always been from Fox."

Baier said on Monday that Fox hadn't changed and that its poll results matched others, adding that the network's reporting is "fair, balanced, and unafraid."

Trump has recently ramped up his war with the network that has long been his ally and is said to be afraid that it isn't loyal enough to him.

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A Fox News host hit back at US President Donald Trump's comment that the network had "changed" after it published a poll that found the president had less national support than four Democratic contenders.

Trump told reporters on Sunday that "Fox is a lot different than it used to be."

"Fox has changed, and my worst polls have always been from Fox," he said. "There's something going on at Fox, I'll tell you right now. And I'm not happy with it."

Read more: Fox News guests and hosts accused the media of 'deliberately' trying to cause a recession to undermine Trump's re-election prospects

The host Bret Baier responded to Trump on Monday and defended the poll, published Saturday.

"OK, well, Fox has not changed," he said, adding that the poll "matches what we're seeing out there."

The poll of 1,013 registered voters earlier this month found that four Democratic frontrunners — Joe Biden, Elizabeth Warren, Bernie Sanders, and Kamala Harris — would fare better than Trump if the 2020 election were held today and they were picked as the party's candidate:

Joe Biden 50%, Trump 38%

Bernie Sanders 48%, Trump 39%

Elizabeth Warren 46%, Trump 39%

Kamala Harris 45%, Trump 39%

Baier defended the results, saying they were similar to RealClearPolitics' aggregation of polls.

He said that Fox's news coverage is "fair, balanced, and unafraid" and distinguished between its news coverage and opinion coverage.

Baier also invited Trump on his Fox News show, "Special Report," noting that Trump has spoken on ABC and NBC shows this year.

Baier. AP Photo/Andrew Harnik

"Mr. President, we've invited you on 'Special Report' many times. We'd love to have you back on. You've talked to George Stephanopoulos and Chuck Todd. Come on back," he said.

Trump has recently ramped up his attacks on Fox News, a longtime ally, reportedly fearing that the network is not loyal enough to him.

Read more: Trump is reportedly worried that Fox News is being taken over by liberals trying to undermine him, as his war on the network escalates

Trump in an interview with Fox News. Courtesy of FOX News Channel's Justice with Judge Jeanine

In July, he tweeted that Fox News was "changing fast, but they forgot the people who got them there!"

He also tweeted earlier in the month that "watching Fox News weekend anchors is worse than watching low ratings Fake News CNN."

Trump still has close ties to the network, however, with some of its biggest names acting as informal advisers to him and its hosts and contributors defending even his most controversial policies.