During a Tuesday interview, Sestak rhetorically asked CNN's Rick Sanchez what Specter's positions were on issues including health care and education. "But, more than that . . . it's reliability," he told Sanchez.
"Will he be with us in 2016? This appears to me to be, unfortunately, more of the political Democratic establishment that made a understandably – but I think short-sighted - decision for expediency here in Washington. What's in it for Pennsylvanians is the question in the long term."
Sestak also said Tuesday that Specter's likely vote with Senate Republicans against the Employee Free Choice Act won't be the sole decisive factor in whether he challenges the incumbent in the Democratic primary.
Sestak, who told CNN Sunday that he wasn't sure whether Specter was a really Democrat yet, also said Tuesday that it was his new colleague's "responsibility to act how he thinks is right. It's our responsibility as Pennsylvanians to judge his actions."
Specter's recent decision to switch to the Democratic Party was immediately followed by pledges of support from President Obama and Senate Majority Leader Harry Reid.