Here is a county-by-county map with the counties Obama carried in blue and those Romney carried in red:

2012 Presidential Election Results by County

Source - Mark Newman, Department of Physics and Center for the Study of Complex Systems, University of Michigan

Here is the same map showing a mixture of Red and Blue to indicate the percentage of votes for each candidate in each county.

2012 Presidential Election Map (Shaded by percent)

Source - Mark Newman, Department of Physics and Center for the Study of Complex Systems, University of Michigan

Here is a similar map that adds a third dimension to show the population of each county:

2012 Presidential Election Results showing County and Population

Source - Robert J. Vanderbei, Professor, Operations Research and Financial Engineering at Princeton Univ.