Gallup Polls taken in 2013, where 18,871 randomly chosen nationwide adults were questioned about their political leanings (and with such a large sample, the poll had only a 1 point margin). This marks the highest figure since Gallup started asking this question back in 1992.
This doesn't surprise me. People are starting to realize that "liberal" is not a dirty word, and they are more and more being turned off by the mean-spirited and hard-hearted policies of the Republican Party (that claims to be conservative). And I think that identification with liberalism will continue to rise as people realize that the beliefs they truly hold are much more liberal than conservative -- views like protecting Social Security and Medicare, providing Medicaid for the poor, protecting the environment, raising the minimum wage, reducing the wealth & income gap, making corporations pay taxes, making the rich pay a higher percentage in taxes than middle class workers, helping the unemployed, and honoring the social contract to take care of the poor and disadvantaged.
Those are all views held by a majority of Americans, but they are not the views of conservative Republicans (especially the teabaggers that now control that party). Those are all liberal views, and as more people realize that, those who identify themselves as liberals will continue to rise. And I think the internet is, and will continue, helping in this -- because this gives liberals a way to spread their message without having to depend on the corporate-owned mainstream media.
Here is the current liberal/moderate/conservative make-up of the Democratic Party, Independents, and the Republican Party: