It’s the 2010 census that makes all the difference. After each census, the Electoral College’s 538 votes are allocated to the states based on population. A simple comparison between the 2000 electoral map and the 2012 electoral map reveals the great effect a census has on the location of electors (click to enlarge):
|Distribution of electoral votes in the year 2000|
|Distribution of electoral votes in the year 2012|
Here's one final map which depicts a hypothetical 2012 electoral map:
Given the current makeup of the Electoral College, Mitt Romney does not need to win all of the states George W. Bush won in order to win the presidency. The map above reveals that Barack Obama could retain Colorado as well as Nevada and Mitt Romney would still win.
What a difference a census makes!