Liberal vs Conservative Identification Hasn't Changed

As pointed out by Kevin Drum, this pair of graphs show that the relative number of people who consider themselves "liberal" or "conservative" has changed very little in the last 20 or so years. In contrast, the relative number of people who consider themselves Democrats has dropped considerably.

This confirms that the declining fortunes of the Democratic Party are not really due to the country becoming more conservative, but due to conservatives leaving the Democrats.