With a $3 million campaign war chest, and a decorated military career behind him, he seemed for a long time to be the Democrats' most compelling potential opponent to either Specter, who was until recently a Republican, or Pat Toomey, who was challenging Specter in the Republican primary from the right.
When Defense Secretary Robert Gates released his Pentagon budget outline this spring, Sestak (a Navy Rear Admiral) played surrogate, defending the plan from critics who were characterizing it as a weak-on-defense spending cut. He was omnipresent. But for the most party, when asked about his political ambitions, he played coy.
Then Specter switched parties--and Sestak's disposition quickly changed. Suspicious of Specter's motives, and unhappy that his once clear path to the nomination was suddenly obstructed, Sestak turned up the temperature on both the Senate's newest Democrat and the party establishment, which wasted no time throwing their support behind the supposed incumbent.
Over the implicit objections of many in his party, he said he'd be undeterred if he ultimately decided to run, and told me recently that Specter would have to move into the Democratic mainstream on a number of issues if he wanted to avoid a primary challenge.
Apparently that move didn't happen swiftly enough.
On MSNBC tonight, he said his official announcement would come in the not too distant future--perhaps a matter of weeks.