From PoliticsPA.com:
Democratic State Rep. Josh Shapiro told PoliticsPA he would not run for Senate "under the circumstances."
"Senator Specter is now the incumbent Democratic Senator," Shapiro said. He was considered a likely candidate for the US Senate.
With Specter's sudden bolt to the Democratic caucus today and the apparent full support of President Obama, Shapiro's decision is hardly a surprise. Next on the list: Rendell ally Joe Torsella, as well as the suburban Philly trio (Murphy, Schwartz, and Sestak).
UPDATE: Joe Sestak is currently refusing to rule anything out:
"I am going to have to wait," Sestak, who has been mulling a run for the Senate, just said on MSNBC. <...>
Sestak said he is waiting "to see what
is running for," implying that he wants to see what Specter will proactively stand for in his campaign instead of just opposing Republican policies.
"If the alternative is Toomey, that's one thing," Sestak said, appearing to indicate that he would support Specter if he is the Democratic nominee.
http://www.swingstateproject.com/diary/4863/pasen-shapiro-wont-challenge-specter