Second, this is a midterm election – incredibly important because of who generally turns out. Those voters are different than ones who turn out in a presidential year; typically, it is smaller, more partisan and more politically engaged electorate.

“This helps Sestak tremendously,” Brown notes. “Essentially, the primary electorate will be made up of loyal Democrats who have most likely voted five times against Specter in general-election contests since 1980. Why in the world would they want to vote for Specter now?”

Specter has a tougher case to make to these folks: It boils down to, "Forget all of our electoral history and vote for me because Rendell and Obama like me."

Sestak's argument is more straightforward and more appealing: "I'm the best Democrat in this primary, so vote for me."

He also is a good fit for the state. He’s a moderate Democrat with a military background. He has a large family that can act as surrogates around the state and help create "personal" connections. And he has a national profile – having served as a national security adviser in Clinton's administration – which will help him raise money in places such as New York and California.

As for his “netroots,” Markos Moulitsas, publisher and editor of the Daily Kos, assures that plenty of money will flow to Sestak once he files.

Should Sestak win both the primary and the general election, he will not owe his seat to the party leadership, allowing him to buck that leadership (if he wants to) on Senate votes.

“That would put him in a great position to be able to craft a legacy that is independent of his party,” notes Brown.

That’s an advantage for Pennsylvanians, enabling him to better represent the state's interests than most senators who make it to Congress.