For days, radio listeners in the Detroit area have been bombarded by ads for Mitt Romney's campaign and spots bought by his allied Super PAC, Restore Our Future. The two have cleaned out the inventory in certain time slots, by some accounts. But even with the air assault, Romney's consultants and staff will be watching nervously Tuesday night as Michigan's returns come in. 

Their eyes will likely be glued to the numbers coming out of the Cadillac-driving Oakland and Macomb counties. These Detroit suburbs are where the former Massachusetts governor has campaigned most heavily -- other than Kent County which is home to Grand Rapids -- and it's where he'll need to put up big numbers in order to win the state. "Those are going to be the tell for Romney," says Bill Nowling, a veteran Michigan GOP operative.

Romney has talked up his campaign's operation in the state. After a woman at a Kalamazoo event complained she'd received nine phone calls from his camp, Romney sounded disappointed. “You should have received a lot more by now," he said.

But the reality, according to state operatives, is that Romney's camp only set up its operation in the last two weeks -- and there have been missteps along the way by advance staff and the candidate. Moreover, there are signs the most organized effort in the primary race is still under prepared for the challenge. On Monday, Romney's campaign was still unveiling endorsements. And the governor himself tried to downplay his odds of winning Michigan.

"We started off about, what, 15 points down in the polls," Romney said, according to the Detroit News, at an event in Albion on Monday. "Now we're leading in the polls. Thanks, you guys!"

For Rick Santorum, who is virtually tied in most polls with Romney, the telling numbers will come out of Kent and Ottawa counties, which surround Grand Rapids. "If he can run up some numbers there, it's going to be close," says Nowling. 

Stu Sandler, a consultant for the state Republican Party, agrees it'll be a tight result. "It seems like Santorum's cutting into Romney's lead in Oakland [County], but it seems like Romney's cutting into Santorum's lead in Kent [County]," says Sandler. "They've both run good campaigns."

Given Romney's deep roots in Michigan and his status as the frontrunner, a close vote on Tuesday will hand Santorum, Ron Paul and Newt Gingrich a mandate to carry on, at least, through Super Tuesday.