I've seen arguments that go both ways on this. On the one hand, there are folks like David Sirota, who argues that they are two ideologies with different methods/approaches:
There is a fundamental difference [between liberals and progressives] when it comes to core economic issues. It seems to me that traditional "liberals" in our current parlance are those who focus on using taxpayer money to help better society. A "progressive" are those who focus on using government power to make large institutions play by a set of rules.
On the other hand, there are folks like Clarence Page of the Chicago Tribune, who argues that the term "Liberal" has been demonized and turned into a "dirty word," thereby leaving Democrats to adopt the term "Progressive" in place of it:
[Hillary Clinton in 2008] lamented that the word Liberal "in the last 30, 40 years" has been "turned up on its head" and "made to seem as though it is a word that describes big government." So, she said, she would rather call herself a "modern progressive" who "believes strongly in individual rights and freedoms" and "working together" to "find ways to help those who may not have all the advantages in life get the tools they need to lead a more productive life for themselves and their family."
This brings about some confusion for me, but I find the latter argument to be more appealing. The term hasn't always had a negative connotation, given that FDR, Eisenhower, and JFK used it proudly. But it has been effectively demonized by right-wingers since the onset of Reagan Conservatism. "Liberal" seems to have taken on a negative meaning of elitists and high-taxers and government bureaucrats. Mondale and Dukakis were successfully characterized this way and the negativity behind the term has continued into today's context and seems to have forced Democrats like Hillary Clinton into using the term "Progressive" instead.
But what say you, NSG?