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

Barack Obama received 816,429 (53%) votes, beating Mitt Romney who gained 727,928 (47%) by 88,501 votes in Iowa.

Mitt Romney won 62 counties whereas Barack Obama won 37 counties in the total of the 99 counties in the State of Iowa.

Presidential General Elections

During the 2012 General elections, over 122M total number of votes were cast. Barack Obama got 62,614,654 (62.6 Million) over Mitt Romney's 59,141,654 (59.1 Million). He won the popular vote by 3,472,979 (3.4 Million). His victory in the State of Iowa (with 816,429 votes) represents 1.3% of his popular vote victory. The remaining 49 states contributed to the remaining 98.7% 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 larger counties (Polk, Linn, Scott), 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

Polk, Linn and Scott, three of the larger counties, voted in favor of Barack Obama giving him a 65k vote advantage. However, Mitt Romney's steady win in the less dense counties (like Dallas, Pottawattamie, Warren and more) was not enough to overcome that difference, making Barack Obama gain an upper hand in the State of Iowa.

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