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

Barack Obama received 2,916,810 (58%) votes, beating Mitt Romney who gained 2,090,120 (42%) by 826,690 votes in Illinois.

Mitt Romney won 79 counties whereas Barack Obama won 25 counties in the total of the 104 counties in the State of Illinois.

2012 General Election Hillary Clinton Popular Vote Count Contributed by Illinois (IL)

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 at the State of Illinois (with 2,916,810 votes) represents 4.5% of his popular vote victory. The remaining 49 states contributed to the remaining 95.5% 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 (Philadelphia, Allegheny, Montgomery), 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

Philadelphia, Allegheny, Montgomery, three of the larger counties voted in favor of Obama, giving him a 611k vote advantage. Lancaster, another larger county voted in favor of Romney giving him a 42k voter advantage in that county. However, Romney's steady win in the less dense counties wasn't enough to catch up, making Obama gain an upper hand in the State of Illinois.

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