Friday’s Muhlenberg tracking poll showed retired Admiral Rep. Joe Sestak taking the lead over recent Republican Arlen Specter in the Pennsylvania Senate Democratic primary. Saturday’s poll, released a few minutes ago, shows that lead growing.

From May 5-8, Sestak lead Specter 46-42 with 12% undecided, almost outside the 5% MOE. From May 4-7, Sestak lead Specter 44-42 with 14% undecided. Just as importantly, in the past week Sestak’s approval has risen 7 points from 45 to 52 while Specter’s has plummeted even more, from 58 to 50.

Regarding the general election, 35% have no opinion of Sestak, compared to just 14% for Specter. Specter had slightly better numbers than Sestak in the last general election poll, but if indie numbers are anything like Democratic numbers, then the undecideds suggest he’s at a November ceiling whereas Sestak still has room to grow. Pollster’s Harry Enten thinks that’s exactly the case, writing on his personal blog Friday that while “it is true that Specter only trails [Repub Toomey] by 6.9% (while Sestak trails by 9.1%), the Sestak vs. Toomey matchup also has a larger pool of undecided voters. Thus, Sestak would have a greater opportunity to pick up voters and close any gap between Toomey and himself.”

Enten’s analysis is of April polls, conducted well before Specter’s terrible, horrible, no good, very bad week. Since then, his popularity has only fallen while Sestak’s has grown. Sestak’s new ad on Specter’s self-serving party switch as well as Specter’s own counterproductive attacks on Sestak’s military service have clearly taken their toll, so one can only assume that the numbers are even worse for Specter now. If another poll validates Sestak’s lead before the May 18 primary, we’ll know the race has a new favorite. There are 3-4 polls in the field now, so we should find out soon.

N = 404 registered Democrats who voted in the 2006 midterm. I haven’t read the question wording, but interviews were conducted by live persons, results are weighted to reflect PA demographics, and respondents were selected at random by a computer process (can’t do RDD since they’re screening only Democrats). Pretty scientific.