A head to head comparison of the 2012 Michigan General Election gives us an idea of the break down of the voters.    

Barack Obama received 2,561,910 (55%) votes beating Mitt Romney who gained 2,112,670 (45%) by 449,240 votes in Michigan.

Mitt Romney won 63 counties whereas Barack Obama won 20 counties in the total of the 83 counties in the State of Michigan.

Presidential General Elections

During the 2016 General elections, over 146,434,269 total number of votes were cast.  Mitt Romney got 59,141,675 (59.1 Million) over Barack Obama's 62,984,405 (62.9 Million). He won the popular vote by 2,865,566 (2.8 Million). His win over Romney in the State of Michigan (with 2,561,910 votes) represents 3.9% of his popular vote victory. The remaining 49 states contributed to the remaining 96.1% popular votes victory.

Top 15 Counties in Primary Elections vs General Elections

All Candidates (General Elections)

These 15 counties play an important role in these Primary elections. If a candidate, can get a sizable majority in the largest counties (Wayne, Oakland, Macomb), it becomes very difficult to win majorities in the smaller counties without voter outreach and increased voter participation in them.

Top 15 Counties Comparison in Primary Elections vs General Elections

General Elections
Barack Obama

General Elections
Mitt Romney

Wayne, Oakland and Macomb, three of the larger counties, voted in favor of Barack Obama giving him a 450k vote advantage. However, steady marginal wins for Mitt Romney in the less popular counties was not enough to overcome that deficit, making Barack Obama gain an upper hand in the State of Michigan.

Related References: 2016 Michigan Primary Election Story, 2016 Michigan General Election Story,  2020 Michigan Primary Election Story