Research 2000 is the most recent pollster to find a small edge for Joe Sestak over Arlen Specter in the May 18 Democratic Senate primary, and also to find Sestak overperforming Specter versus Toomey in the general election. They also find Sestak with by far the most upside of any of the three candidates: Specter's favorables are at 45/44 with 11% no opinion, Toomey's are 45/40 with 15% no opinion, and Sestak's are 39/26 with 35% no opinion.
Muhlenberg College for Allentown Morning Call (pdf) (5/10-13, likely voters, 5/9-12 in parentheses):
Dan Onorato (D): 39 (39)
Anthony Williams (D): 14 (14)
Joe Hoeffel (D): 11 (11)
Jack Wagner (D): 11 (9)
Undecided: 25 (27)
(MoE: ±5%)
Muhlenberg finds things going the other direction, with a move from a tie in its daily tracker to a 2-point lead for Specter. They also find almost no movement in the gubernatorial primary, with Dan Onorato still in a dominant position.