A head to head comparison of the 2008 Iowa General Election gives us an idea of the break down of the voters.    

Barack Obama received 828,940 (54%) votes, beating John McCain who gained 682,379 (44%) by 146,561 votes in Iowa.

Barack Obama won 53 counties whereas John McCain won 46 counties in the total of the 99 counties in the State of Iowa.

Presidential General Elections

During the 2008 General elections, over 131M total number of votes were cast. Barack Obama got 69,448,278 (69.4 Million) over John McCain's 61,739,059 (61.7 Million). He won the popular vote by 7,709,219 (7.7 Million). His victory in the State of Iowa (with 828,940 votes) represents 1.2% of his popular vote victory. The remaining 49 states contributed to the remaining 98.8% 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 (Polk, Linn, Scott, Johnson), 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
John McCain

Polk, Linn, Scott and Johnson, four of the larger counties voted in favor of Barack Obama, giving him a 98K vote advantage. However, steady marginal wins for Barack Obama in the less popular counties (like Story, Clinton and more) were enough to maintain that difference, making him 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, 2012 Iowa General Election Story