Democrats have lost their solid political party affiliation advantage in 18 states since 2008, while Republicans have gained a solid advantage in 6 states. A total of 17 states were either solidly Republican or leaning Republican in their residents’ party affiliation in 2011, up from 10 in 2010 and 5 in 2008. Meanwhile, 19 states including the District of Columbia showed a solid or leaning Democratic orientation, down from 23 in 2010 and 36 in 2008. The remaining 15 states were relatively balanced politically, with neither party having a clear advantage…

The findings make it clear that U.S. states have undergone a dramatic political transformation since 2008, the year President Obama was elected, moving from a Democratically dominant political environment to one of parity.

The greatest movement away from the Democratic Party came between 2009 and 2010, when the number of states with a Democratic advantage fell from 34 to 23.