More than 2.5 million voters have left the Democratic and Republican parties since the 2008 elections, while the number of independent voters continues to grow.

A USA TODAY analysis of state voter registration statistics shows registered Democrats declined in 25 of the 28 states that register voters by party. Republicans dipped in 21 states, while independents increased in 18 states.

The trend is acute in states that are key to next year’s presidential race. In the eight swing states that register voters by party, Democrats’ registration is down by 800,000 and Republicans’ by 350,000. Independents have gained 325,000.

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