Rasmusssen (and other pollsters) ask people to self-categorize themselves by political party.
I think the trends are pretty interesting:
During the final years of the Bush admin, the mix shifted from the GOP to independents. Note the near mirror image of the red and green lines on the left half of the chart.
When Obamamania caught traction, the mix shifted from independents to Democrats. Again, not the near mirror image of the green and blue lines on the right half of the chart.
Now, the mix is shifting again — from Dems to independents.
While Dems still have a statistically significant plurality, the country broken roughly in thirds across Dems, GOP and independents … with independent “swing voters” carrying determining sway.