The NFL rumor mill has kicked into high gear regarding San Francisco 49ers head coach Jim Harbaugh.

The latest round of rumors says that barring a deal with the University of Michigan, the Oakland Raiders and Chicago Bears are expected to make serious runs at hiring Harbaugh away from the 49ers.

"If UofM doesn't land Harbaugh quickly, the Bears and Raiders are expected to be aggressive. And this thing is trending to happen quickly," tweeted Albert Breer of NFL Network midday Sunday.

ESPN NFL Insider Adam Schefter reported shortly thereafter that the Raiders will make a "concerted effort" to land Harbaugh.

The Raiders interest in Harbaugh has been known for some time. He's the type of gritty taskmaster Oakland may need in order to fully change the culture surrounding the organization. With some strong building blocks in quarterback Derek Carr and outside linebacker Khalil Mack, Harbaugh could build the team anew in his image.

For the Bears, a team completely devoid of leadership this season, Harbaugh could be a natural fit to alter the outlook in the Windy City - his personality and aggressive style would be almost the complete opposite of what they had this season in Marc Trestman.

But both teams will have to act fast if they're to have a real chance at acquiring Harbaugh.

Breer also said that the 49ers are still very much unsure of what Harbaugh's plan will be once the season ends and that, while Oakland and Chicago have reinserted themselves into the conversation, Michigan is very much in the lead to land the aggressive head coach.

"Coaches in San Francisco are still waiting to hear definitively what Jim Harbaugh will do. But there's no question Michigan is the lead dog," he tweeted.