There has been a noticeable increase in the number of self-identified liberals over the past decade. According to Gallup, back in 2002 only 19 percent of Americans considered themselves to be liberals while now that number is 23 percent. From Gallup:
The growth in liberals is coming at the expense of so-called moderates. Since 2002 moderates went from 39 percent of the population to only 34 percent.
We are becoming a more ideologically divided country and that in turn is making our politics more partisan.