It's been just over a week since Sen. Arlen Specter (D-PA) switched parties, and already progressive activists are taking the temperature in Pennsylvania to see if the climate's right to retire him. The liberal Progressive Change Campaign Committee, in partnership with some of the Netroots' most visible blogs, is conducting a week-long straw poll to determine both the level of progressive grassroots support for challenging Sen. Arlen Specter (D-PA) in the 2010 primary, and whether the challenger should be Rep. Joe Sestak (D-PA).

The development resembles in some ways the early days Ned Lamont's successful campaign--backed by the progressive grassroots--to defeat Sen. Joe Lieberman in the 2006 Connecticut primary. Lieberman, of course, turned around and beat Lamont in the general election by running as an Independent, but election laws in Pennsylvania would prevent Specter from doing the same thing.

For his part, Sestak seems to be positioning himself to become Netroots darling. In the last week he has grown more and more critical of both the party establishment for thoughtlessly backing Specter, and of Specter himself, who he says is an unreliable representative for Pennsylvania Democrats.