I really don't think there was a lot of crossovers. The polls say otherwise as did the hype, but the results show that most dem voters went with Obama or uncommitted.
As for Santorum Crossovers, I know he did well on election day in white social conservative union areas, but there wern't a lot of hard core D crossovers. Santorum did best in the middle class and working class Republican areas. Romney did best in the higher money republican areas. Genoa 8 (my parents precinct went almost 3-1 Romney. Most of that was Pine Creek.
The 29th district:
Auburn Hills:
Romney - 40.56% - 743
Santorum - 34.88% - 639
Paul - 15.23% - 279
Obama - 782
Dem uncommitted - 53
Pontiac
Santorum - 575 - 41.01%
romney 364 - 25.96%
Paul - 317 - 22.61%
Obama - 4883 -
Dem Uncommitted - 2.28%
Really not a lot of crossover. Pontiac is about 80% Dem. Auburn Hills about 55-65% Democrat.
51st District Area
http://www.gc4me.com/departments/county_clerks1/docs/Elections/Feb2012/12FEBPRI_CANVASS.pdf
Quick looks:
Santorum won Argentine Twp
Romney won Atlas (narrowly), Fenton (Big), Grand Blanc, Linden, and Mundy Twp. Some of those were thanks to Absentees, like Atlas (strong GOP) and Mundy (leans dem, but winnable)
If there was 100,000 democrat votes for Santorum, I don't see it. I'm looking at Ingham County right now. Santorum's doing "well" in Lansing, but those precincts are all small and those are close. Those also could be Republicans as well who may be more social conservative. East Lansing would be a better test for crossovers. Romney did much better than Santorum, and some of the student areas went for Ron Paul. Obama may have even won it. He took a couple precincts 37-25-20, but is that significant? Meridian Township - Romney easily. Santorum won Webberville and some of the rural areas.
I think crossovers may have factored in the 13th district, but places like Westland, Redford, and Garden City would be more friendly to Santorum anyway than other areas in Wayne County. Plymouth/Northville and Grosse Pointes aren't Santorum territory.