In the otherwise 50/50 counties, Santorum lost absentee voters 2-1. That was where this election was decided. Romney won Macomb County by 200 on election day. He won the absentees by over 7000 votes.
Santorum's support didn't "collapse" on election day. He probably WON on election day. He did not have 55% to begin win in early February because the absentee vote destroyed him.
Mitt Romney beat Rick Santorum because he did best with seniors and had a good absentee program going while Santorum just got started campaigning in Michigan. It wasn't total loss because Santorum got 14 out of the 16 delegates.
14 delegates to Romney’s 16.
You’re wrong, Santorum’s support did collapse on the last day. You are right that Romney had a lead from the very earliest deciders but that switched to Santorum in February. You need to look at the exit poll:
http://www.cnn.com/election/2012/primaries/epolls/mi?hpt=hp_t1
Scroll down to the “Vote by Time of Decision” section.
Romney’s lead in absentees was probably from people who decided in 2011. A whopping 32% of them did and Romney won them 53% to 16%.
Romney just barely won people who decided in January.
Santorum won big from everybody who decided in February up until the last day. He was in the 50s, Romney in the low 30s. Santorum began losing steam in the last few days though.
Then suddenly Santorum collapsed on the last day. His support went from 50% to 31%, putting him way under even his January average of 41%. The votes that left him split evenly among Newt, Mitt and “Uncommitted,” whatever that means in an exit poll.
I do believe exit polls do incorporate polls from people who voted absentee.