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
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)
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