“Oftentimes, people want to make politicians perfect,” she added. “And that’s one of the actual beauties of Christianity, is understanding that no one is.”

She is the first mother to serve as press secretary, and among the youngest to occupy the role. At the lectern, she is folksy, but nimble: She recently deflected questions about “chaos” in the White House by inviting reporters to visit the three preschoolers in her living room. There were laughs, even as Ms. Sanders clearly sidestepped the subject.

She is also unafraid to call out reporters and news coverage that she deems unfair. Asked in the interview if the establishment media is biased against Mr. Trump, she replied, “Absolutely.”

“I’ve never seen the level of hostility that this press corps has to the president,” she said.

Behind the scenes, reporters who cover the West Wing say Ms. Sanders can be friendly and warm — the good cop to Mr. Spicer’s barking sergeant. Last week, several dozen journalists and White House aides, including Kellyanne Conway, toasted Ms. Sanders’s promotion at an all-female “women of the White House” happy hour at a Washington hotel bar.

But like Mr. Spicer, Ms. Sanders has drawn criticism for some dubious assertions. Confronted with Mr. Trump’s call for law enforcement agents to rough up gang suspects, she said that the president “was making a joke.” While denouncing CNN, she urged Americans to watch a video critical of the network by a right-wing activist, James O’Keefe, “whether it’s accurate or not.”

When a reporter asked if Mr. Trump had lied about a laudatory phone call from the Boy Scouts, Ms. Sanders shot back: “That’s a pretty bold accusation.” She also conceded that the call had not happened.