Mitt Romney really thought he was going to defeat President Barack Obama in the 2012 election — a belief he held right up until the polls closed on Election Day. As he told reporters on his campaign plane en route to his election night rally in Boston, he hadn’t considered losing. He hadn’t even written a concession speech.

And that wasn’t just rhetoric. A new documentary about Romney’s 2008 and 2012 presidential bids captures the moment the GOP nominee realizes his quest for the White House is over, that the internal poll numbers that suggested a landslide victory over Obama were dramatically wrong.

“I just can’t believe you’re gonna lose,” someone tells him.

“Yeah, yep,” a stunned Romney replies.

“So what do you think you say in a concession speech,” Romney adds with a hint of a smile. “By the way, someone have a number for the president?”

It’s the opening scene from a trailer for “Mitt,” a documentary set to premiere at the Sundance Film Festival on Jan. 17 and stream on Netflix starting Jan. 24.

The film, directed by Greg Whitley, features behind-the-scenes footage and candid interviews with Romney and his family, dating to 2006 when the former Massachusetts governor ran and lost his bid for the GOP nomination to Sen. John McCain.

The documentary shows another side of Romney, who was long criticized for being awkward and forced on the stump. In the film, Romney is joking and at ease — as he often was during off-the-record chats with reporters on his campaign plane. One scene shows him trying to iron a suit while he’s actually wearing it.

Another scene shows him bluntly acknowledging the public regards him as “the flipping Mormon” — a rare admission of the effect his religion had on his bid for the presidency.

The film also includes footage of the Romney family discussing his potential presidential bid ahead of the 2008 campaign and Romney’s blunt admission that the nominee who runs and loses a bid is branded “a loser for life.”

“If you don’t win, we’ll still love you,” his oldest son, Tagg, tells him at one point. “The country may think of you as a laughingstock, and we’ll know the truth, and that’s OK.”