Summer is not the only thing sliding away. When Congress returns next week, Democratic leaders will get an earful of advice from rank-and-file members about how to stop their political slippage. But it’s unclear whether much can be done. That’s because independent voters are the catalyst behind Democrats’ falling numbers. And these Americans are on the move–away from the Democrats–due to several reasons difficult for party leaders to control.

The recent shift should shake Democrats to their core. Winning this group in American politics today is critical. People who call themselves “independents” represent about a third of the electorate. After closely dividing their votes between the two parties in 2000, 2002, and 2004, independents gave Republicans the cold shoulder in 2006, beginning a two-election cycle ice age with this critical electoral bloc. The big chill cost the GOP its congressional majority three years ago and helped elect Barack Obama president last fall.