The last time Oscar voters surprised with a best picture winner came eight years ago when Jack Nicholson opened the evening’s final envelope, arched those famous eyebrows and announced “Crash.”

He then mouthed the word “Whoa!”

Most years, Hollywood insiders have a good idea who will go home with an Academy Award. By the time the Oscars are handed out at the end of a long awards season, clear favorites have emerged from earlier contests.

Going into Sunday’s 86th Academy Awards, there are front-runners in the acting, directing and writing categories. But there’s one big exception: best picture. That contest has turned into a three-way battle among “Gravity,” “12 Years a Slave” and “American Hustle.”

The three leading contenders have engaged in a game of awards-season musical chairs since December. “American Hustle” won the top prize from the New York Film Critics Circle. “Gravity” found favor with the Los Angeles Film Critics Assn. “12 Years” and “Hustle” each won best picture awards at the Golden Globes in January.

A week later, “Hustle” took the Screen Actors Guild ensemble award, while the Producers Guild’s top prize ended in an unprecedented tie between “Gravity” and “12 Years.”

Never before have the major guilds had a three-way split for their top honor, producing a best picture race so tight that the Academy of Motion Picture Arts and Sciences took the preemptive step of announcing last month that its balloting system guarantees one winner and one winner only.

“There’s going to be genuine suspense this year when that final envelope is opened,” says veteran studio awards consultant Tony Angellotti, who doesn’t have a horse in this year’s race. “And that hasn’t happened in some time. ‘Argo,’ ‘The Artist,’ ‘The King’s Speech’ … everyone knew those movies were going to win.”

The uncertainty stretched the contenders’ campaigns all the way through Feb. 25, the deadline for the 6,028 academy members to turn in their Oscar ballots. Interviews with dozens of academy members in the last week suggest there is no clear favorite among the top three contenders, with the outcome likely hinging on which one lands the most second- and third-place votes.

The academy uses a preferential voting system, with members ranking each of the nine nominees one through nine. Movies with the fewest first-place votes are eliminated, with their ballots shifted to next highest-ranked film. The reallocating continues until one movie owns 50 percent plus one of the voting.

This system, which the academy adopted when it expanded the best picture category in 2009, rewards movies that enjoy a broad consensus. It’s one reason why many pundits believe the crowd-pleasing outer space survival story “Gravity” will prevail over “12 Years a Slave,” a harrowing look at a free man sold into slavery in pre-Civil War America that includes numerous scenes of brutal punishment.

“I still have friends who haven’t watched the movie,” says one voter who spoke on condition of anonymity (the academy frowns on members speaking publicly about films in contention).

The three studios in the hunt have spared no expense to keep their movies in academy members’ minds. The campaign spending has at least equaled the record amount last year when the deep-pocketed studio backers of “Argo,” “Lincoln” and “Life of Pi” spent $10 million and upward on best picture campaigns, industry consultants say.

Most of the year, movie ads are designed to sell tickets at the box office. But the big ads for Oscar-nominated films running in the Los Angeles Times, New York Times and other media outlets during the 12-day Oscar voting window were aimed squarely at academy members who read those publications.

During the final balloting period, Fox Searchlight ran ads for “12 Years” featuring lead actor Chiwetel Ejiofor and the words “It’s time.” The not-so-subtle message: “It’s time” for Hollywood to honor a film that takes an unflinching look at American slavery.

Reaction among academy members has been mixed. Some have lauded the studio for what they see as a bold campaign. One academy member, referring to speculation that many voters have been reluctant to screen their “12 Years” DVD, said the ad might even have a secondary message: “It’s time to man up and watch the damn movie,” she said.

Others have viewed the ads as a cynical attempt to shame academy voters – who are predominantly older white men – into casting their ballots for “12 Years.”

“Vote for the black movie to win because it’s time to do so?” asked one voter. “That’s offensive.”

TACKLING A SENSITIVE SUBJECT

While reactions to the “12 Years” campaign have been all over the map, the ads did maintain the consistent message that “12 Years” merited strong consideration because of its courage to tackle a sensitive chapter of American history that few other movies have examined.

The film’s director, Steve McQueen, attended numerous events that did not include academy members to spread the true story of slave Solomon Northup.

“At the beginning of all this a friend told me, ‘Steve, this picture is more important than you,’ ” McQueen said after the event. “And it is. People are seeing a time in history that relates to them in the present day and the future.”

The London-born McQueen concedes that campaigning isn’t his “cup of tea.” That attitude represents a marked contrast to the friendly Alfonso Cuaron, his rival for both the director and picture Oscars.

The Warner Bros. campaign for “Gravity” has reflected the image of its down-to-earth director, spotlighting the movie’s innovative technology while trying to emphasize that it was created to serve the emotional journey taken by Sandra Bullock’s stranded astronaut. Though that message has been lost on some voters (“You’ve forgotten the story by the time you’ve reached your car,” said one), Cuaron has steadfastly asserted it since the Telluride Film Festival in September.

“I never thought about making a groundbreaking film,” Cuaron said. “It was just the only way to make this story about a woman overcoming adversity.”

David O. Russell, Oscar-nominated for director and original screenplay for “Hustle,” might have been the most ubiquitous presence on the campaign trail since nominations were announced in January, constantly donning one of his five identical J. Crew three-piece suits to talk about “Hustle” and his filmmaking career.

“How can you complain?” Russell said recently at a benefit event he did for the Santa Monica video store Vidiots. “And you know, if I have to sit on my behind for five hours at an event and watch other people win, so what? I’m just grateful to be included.”

The rewards for the winner go beyond bragging rights. A best picture Oscar helps sell more tickets if the movie is still in theaters, and DVDs down the road.

It also cements the legacy of both film and filmmaker, much as a Super Bowl victory does for a winning quarterback. (Ask Peyton Manning.)

“It’d be nice, sure,” Russell says, musing on an Oscar win. “But you see that shelf over there,” he adds, pointing to a rack of DVDs in Vidiots bearing his name. “To me, that’s almost as heavy as an Oscar.”