Tagovailoa opened with the third-best odds (10-1) to win the Heisman at the Westgate Las Vegas SuperBook in June, and is currently the co-favorite at 5-1 with Stanford running back Bryce Love.
Earlier in the week, Tagovailoa was the outright favorite at 4-1.
"Tua has been very popular," SuperBook director and oddsmaker John Murray told ESPN. "He's our biggest liability right now, along with McKenzie Milton [quarterback at Central Florida who is at 50-1]."
Westgate SuperBook manager Jeff Sherman told ESPN that Tagovailoa -- who currently is battling Jalen Hurts for the starting job -- accounts for over 21 percent of all money wagered on the Heisman at its sportsbook.
Tagovailoa is not the consensus favorite around Nevada sportsbooks, though, as Caesars has Wisconsin running back Jonathan Taylor as the favorite, with about 20 percent more bets than Love and Penn State quarterback Trace McSorley.
Love is the 4-1 favorite and has attracted the most money at MGM's Nevada properties, while West Virginia QB Will Grier has the most tickets at 15-1. Love is the favorite at South Point (6-1), with Tagovailoa and Taylor at 7-1.
Alabama has a regular-season win total of 11.5 games at Westgate, meaning the Crimson Tide would need a 12-0 regular season to cash. The Crimson Tide are 24.5-point favorites in their opening game Saturday against Louisville in Orlando, Florida.