The managing editor of Fox News picked up the phone in the room where he and others were looking at election returns. The control room was on the other side.

Mr. Sammon told the producers and executives that the network was calling for Joseph R. Biden Jr. to be the next president of the United States. Arizona was blue on the map that viewers saw at home when he clicked a box on his computer.

The moment unfolded without much drama inside Fox News. The result was obvious to the people in the room. It provoked a fury with President Trump and his supporters who attacked Fox News as dishonest and disloyal.

The former president and the network wouldn't be the same.

On Monday, the events of that night were the focus of a congressional hearing. The testimony from the former senior editor at Fox News was part of the House investigation into the Capitol riot.

Chris Stirewalt, who was the politics editor for Fox News until he was fired two months after the election, told the House committee that he knew Trump had a small chance. The cautious approach they took to determining that Mr. Trump couldn't overtake Mr. Biden in Arizona was described by Mr. Stirewalt.

He said that they took a vote from the people who worked on the decision desk. After the group agreed, Mr. Sammon issued it.

The room was looked at. Everyone says yes. Mr. Stirewalt told the committee that they had already looked at calling other states when they heard of the backlash they had caused.

The second televised hearing by the committee aimed to refocus the country's attention on the horrors of that day and to make a compelling case that Mr. Trump continued to lie about voter fraud and "stolen" votes.

The hearing on Monday centered on people who said they did not believe in the validity of the former president's claim that he won the election because of the early vote returns.

The "Red Mirage" was the subject of an issue. As polls closed across the country, Mr. Trump was widely expected to appear far ahead, because the first votes were cast from people who voted in person. Political experts warned that that would be a terrible thing. They said that Mr. Trump's lead would shrink as states counted the mail-in votes.

Several weeks before the election, a group of advisers, including Stephen K. Bannon and Rudy W. Giuliani, encouraged Mr. Trump to declare victory on the night of the election, arguing that he could easily dismiss mail-in ballots as rife with fraud regardless of whether he had any evidence.

The Arizona call blew a hole in the strategy. A bad night was predicted by a projected loss in traditionally red Arizona for the Democratic presidential candidate.

Fox News was the only news outlet prepared to make a call at that point in the evening, and they had good reason to be confident about it. Other networks did not have the decision desk's data.

Murdoch urged Fox to pull out of the group of news organizations that used polls after the election. The polls were wrong in predicting a Hillary Clinton victory.

That paved the way for Fox News and The Associated Press to go their own way in 2020. In the weeks leading up to the election, they surveyed 100,000 voters who had cast ballots early and gave them a sense of how misleading themirage might be. The Fox News decision desk compared the surveys with the actual precinct-level vote tally that the A.P. was tracking.

The A.P.-Fox News project worked well according to Mr. Stirewalt. He said that the poll in Arizona was gorgeous. It was doing what we asked it to do.

Some of Mr. Trump's former aides testified that the Fox call undermined their confidence in his ability to win the election. In video testimony played by the committee, a senior aide to the Trump campaign said that he and others were disappointed with Fox for making the call, but at the same time they were concerned that the data or numbers they were using weren't accurate.

Mr. Miller didn't share that concern on election night when he said that Fox should be ignored by other media. Mr. Trump demanded an explanation from Fox executives, producers and on-air talent. The son-in-law of the president called Murdoch. The scene played out on the air as Fox talent commented on the complaints coming in from the Trump campaign.

"Arnon, we're getting a lot of incoming here, and we need you to answer some questions," the network's chief political anchor, Bret Baier, said at one point.

The decision desk's process did not include Mr. Murdoch or Lachlan Murdoch. The Murdochs were not involved, according to network executives.

The decision desk, which is a separate part of the news-gathering operation and is overseen by a polling expert who is also a registered Democrat, has never adopted the idea that Fox News coverage is usually favorable to conservative, pro- Trump points of view. Mr. Mishkin was steadfast in his defense of the call in the days after the election. The host Martha MacCallum asked Mr. Mishkin what if the frog had wings. The decision desk for the upcoming elections will be run by Mr. Mishkin, who is a paid consultant for the network.

The decision desk was created by Roger Ailes, who was fond of making controversy and drawing ratings more than he cared about toeing the line for the Republican Party. In 2012 it was the first to predict that President Barack Obama would win Ohio and a second term, and in the same year it predicted that Republicans would lose the House of Representatives even as votes were still being cast on the West Coast.

The Murdochs paid a price for the Arizona projection.

The crowd at Mr. Trump's rally chanted, "Fox News sucks," prompting the former president to urge his supporters to switch to Newsmax and One America News Network.

Newsmax saw a ratings increase as Fox showed some rare slippage.

Soon, various Fox opinion hosts were giving oxygen to false assertions that the election was stolen, including one former Trump aide who called themnuts.

The journalist who defended the Arizona call was fired by Fox News.