Casey Affleck has been the odds-on favorite to win best actor at the Academy Awards for the past several weeks, but on Sunday night, he lost the most predictive pre-Oscar award in his category, the Screen Actors Guild Award for best male lead actor. A full 82 percent of the past 22 winners of that prize went on to win the Oscar. The next most predictive award based on our points-for-predictive-wins model — the British Academy of Film and Television Arts prize — has an accuracy rate of 52 percent over the past 25 years.

Affleck benefited from one of the two heaviest Oscar advertising campaigns in the industry press during the nominations period, according to a FiveThirtyEight analysis of “for your consideration” ads — Affleck’s film, “Manchester by the Sea,” is produced by well-heeled newcomer Amazon Studios. And until Sunday, Affleck enjoyed front-runner status in our model thanks to wins at the New York and Chicago critics awards, the National Board of Review, the Critics’ Choice awards and the Golden Globes. But all of those combined aren’t enough to make up for his loss at SAG to Denzel Washington (“Fences”), who is now in the lead.

Oscar bettors are not hitting the lifeboats on Affleck yet, but his chances have taken a serious plunge. Since Jan. 7, I’ve been tracking the changes in the odds of films and actors winning an Oscar in the acting, directing and best picture categories on Irish gambling site Paddy Power. Affleck’s loss at SAG was the second-biggest hit to win probability that any nominee has seen this season. He went from an 83 percent implied chance of winning to 64 percent by Tuesday morning. That 19-point swing is huge but doesn’t change the fact that he’s still the bettors’ favorite. It does, however, mean that he’s much closer to square one, when he had even odds going into the Golden Globes. Compare that with the steady ascendancy toward “Oscar lock” status that he’s enjoyed over the past several weeks.

The real issue for Affleck is if Andrew Garfield (“Hacksaw Ridge”), Viggo Mortensen (“Captain Fantastic”) or Ryan Gosling (“La La Land”) wins at the upcoming BAFTA awards (Washington isn’t nominated). That would suggest that the film’s Oscar campaign isn’t making a dent and that insiders are truly looking elsewhere. It’s a far more tenuous position than he was in on Saturday.