Gallup has an interesting new statistic: 42% of Americans self-identify as Independents. Republican ID is down to 25% and Democrat to 31%. They also posit that most of that attrition is at the expense of Republicans.
Republicans, hit a recent high of 34% in 2004 and have basically fallen since then. Democrats hit their high in 2008 with 36%. The 2004 GOP high was on the heels of GWB's successful re-elect and the Democrat high ran in tandem with the Hope-N-Change election of 2008.
Truly, there is no mistaking a growing dissatisfaction with the two major parties. Dissatisfaction with Democrats because they are becoming more and more progressive with little tolerance or patience for any politician that isn't hard left (remember Tim Holden?) and dissatisfaction with Republicans after they abandoned positions of individual empowerment, freedom, and fiscal restraint to become Democrat-Lites.
A major policy correction in the GOP occurred in 2009/2010, giving them sweeping victories in 2010 and started their return in popularity...until it became clear that they could neither deliver on their promises (largely due to only capturing the House) nor effectively communicate with the American people.
If Republicans want to register new voters, they must pursue policies that empower individuals to succeed, protect individual freedoms, and return true fiscal sanity to government but they also must be fearless in advocating for these policies. (Note to GOP: the mainstream media is not going to support you or your efforts, deal with it by talking directly to the people.)
The Democrats, on a similar trajectory as the 2005/2006 GOP, will be forced to go through a major policy correction, as the Republicans did, or will continue to hemorrhage centrists until they find themselves in a similar position.
Before Democrats can attract new voters, they must resolve their identify crisis. Will they be the party of "from each according to his ability, to each according to his need" or will they once again realize their primary challenge is "not how to divide the economic pie, but how to enlarge it."
Until both major parties figure this out, the ranks of Independents will continue to swell.