NFL chalk is on the verge of history this week.

NFL betting favorites — known as the chalk — have won 183 games outright this season, five shy of the all-time mark set in 2005, with Week 18 pending. The favored team has won 71.8% of games entering the final week, the highest winning percentage for chalk since 2005 and second-best since the NFL adopted a 16-game schedule in 1978, according to ESPN Research.

Sportsbooks got off to a hot start to the season, with underdogs springing 26 upsets in September, the second-most ever for the opening month. Nevada sportsbooks won a net $80.9 million in September, the most lucrative month in the state’s 75-year betting history.

But the upsets dwindled during the final three months of the season, and favorites have been cooking in December. Favorites won as many as 15 of 16 games in Week 17 (the point spread varied on the Green Bay Packers–Minnesota Vikings and Tennessee Titans Jacksonville Jaguars games). Overall in December, favorites are 56-17 straight-up, a 76.7% win rate, the second-highest for the month in the last 50 seasons.

The run of favorites has been a boon for the betting public, which typically backs the perceived superior team, especially in parlays. Chris Andrews, a veteran Las Vegas sportsbook director with the South Point, estimated his shop needs the underdog in 75% of NFL games.

“Maybe more this year,” Andrews told ESPN in a text message.

Favorites have covered the spread in 53.9% of games this season, the best rate since 2017 and seventh-best in the Super Bowl era.

“Some of our seasoned traders have said this is the worst NFL season since 2017,” Joe Brennen, head of Prime Sportsbook, told ESPN.

Caesars Sportsbook vice president of trading Craig Mucklow said, with so many favorites winning, bettors were in position for a big weekend.

“The biggest decisions all went the way of the customers in the Chargers and Bengals [on Saturday],” Mucklow said. “The parlay buster in the New York Giants outright win [over the Indianapolis Colts] was the difference between a historic day for the customers versus a huge sigh of relief for the book.”

Week 18 features an unusual underdog in the two-time defending-champion Kansas City Chiefs. The Chiefs began the week as 10-point road underdog to the Denver Broncos, who are playing for their playoff lives. Kansas City has clinched the No. 1 seed in the AFC.

“Maybe an underdog can win a football game?” John Murray, vice president for the Westgate SuperBook, said. “That would be cool.”