Chris Stirewalt defended his team’s calling the election for Biden. Fox News fired him.

When Chris Stirewalt watched rioters attack the Capitol on Jan. 6, 2021, he was a journalist for Fox News, which was trying to recover from a ratings slump in the weeks after President Donald J. Trump lost the election.

Two months later, Mr. Stirewalt was fired by Fox, where he had been a regular on-air presence as the politics editor.

The network gave no public reason for his dismissal. But Mr. Stirewalt, who before his ouster was one of the shrinking number of news journalists left at Fox News, was on the team that had decided to call Arizona for Joseph R. Biden Jr. shortly before midnight on Election Day in 2020, effectively declaring the race over days before the results would be settled.

On Monday morning, Mr. Stirewalt will testify before the House select committee investigating the Jan. 6 attack. He is expected to discuss how he and the members of the Fox News Decision Desk relied on hard data to make that call, which deeply angered Mr. Trump and blunted his ability to falsely claim that he had won.

In the days after the election, Mr. Stirewalt was one of the Fox personalities who went on the air to defend the call as Mr. Trump attacked the network and his supporters voted with their remotes, switching over to Fox rivals like Newsmax and the One America News Network.

Mr. Stirewalt described the decision as the product of a cautious and rigorous internal process. “We are careful about making calls,” he said the day after the election. “That’s why we have those protocols in place so that we make good calls and that they stand up.”

Since his ouster, Mr. Stirewalt has become an outspoken critic of his former employer and what he has described as an information bubble that is doing a disservice to Trump supporters. He has called the notion that fraud cost Mr. Trump the election a “lie” and said that the former president’s initial success was in part the result of the “informational malnourishment” of his supporters.

In an opinion article for The Los Angeles Times that he wrote shortly after departing Fox, Mr. Stirewalt said he was hopeful that the truth would prevail. But he acknowledged that after watching what happened on Jan. 6, he had doubts as to whether that would “come quickly enough when outlets have the means to cater to every unhealthy craving of their consumers.”